Lacks power at end of gear, its like a old 4 stroke, no power band and just goes at a steady speed.

Tuning or modifying a 2-stroke engine needs a wealth of experience for best results and is almost a type of witchcraft.

Small things can make a difference to the power output and to where in the rev range the power happens, the more an engine is modified the narrower the band where the power is to be found. The exhaust has a huge influence and a half-inch either way in the dimensions of an exhaust silencer/expansion box can move the power band 500 rpm, for instance.

Some of the 2-strokes from my history hardly developed enough power to keep themselves running until until several thousand rpm is achieved and then the power makes a sudden hit.

Carburettor tuning is similarly complex with some carbs requiring much more than alternative jetting for success, with things like throttle slide cut-outs and needle profiles to consider.

Fitting a larger carb means the induction pulse at low rpm might not be sufficient to produce fine fuel droplets for efficient burning. The induction shock waves could be forcing fuel back out of the carb at certain speeds, fuel that can be lost to atmosphere unless contained by a ram pipe.

Without the understanding and experience to discover, analyse and overcome these difficulties, engines perhaps do not achieve capability and often use far more fuel than they should, as the layman's answer to these things is usually over-jetting.

I suggest you seek out a serious specialist for more specific advice and information.

4.11 gear ratio front and rear diffs, did lots of these over the years.

Check the linkage firs if it is in the right place then you will need to rebuild the trans or get a new one as the bands are gone in it. Try to change the fluids and filter first. But from the sounds of it i would say the bands are gone and it needs a new trans.

A 2-stroke engine with an expansion box tends to have a narrow power-band, a much narrower band than with a stock exhaust and outside of that narrow band often an engine hardly develops enough power to keep itself running. Earlier 2-strokes are particularly "peaky" with a tuned exhaust and must be ridden accordingly.

The design and dimensions of a tuned exhaust can be tweaked to produce different power-bands at different speeds but it is all almost in the realms of the black arts. A half-inch here or there can move the band by 500 rpm.

Rejetting might improve matters or might not but riding above 2000rpm shouldn't take much effort and in the world of 2-strokes is fairly normal.

I suggest you do some reading on the subject and the Haynes manual is a good place to begin. The link below leads to the second edition which I haven't read but I think the first edition would probably be better if you can find one.

https://haynes.com/en-gb/two-stroke-performance-tuning-2nd-edition

Check fuel filter and fuel pressure.

apart from the obvious check air filter is clean spark plug is good and your pilot jet in carb is all clear form dirt or gummed fuel my next step would be put a tooth or two bigger rear sprocket on it to lower the gear ratio this will give more torque low speed maybe whilst you looking at the sprocket just check that some has not change the front to smaller sprocket it being few years old some one may have set to get more top end speed and sacrificed all the low end??

Either the shaft coming out of the transmission housing to the shift lever is stripped out, the cinch bolt on the shift lever is very loose, or the transmission is shot. Examine closely and take to the shop if needed.

this easy, drain oil from gear box remove that drive shaft (u need a ball joint splitter)
replace gearbox seal, pry out old 1 with screw driver, gently tap new in, oil edge of new seal 1st. replace oil, ps dont leave to long as will damage gearbox when lack of oil

check for a clogged exhaust look at catalytic converter it could be stop up.

That is so dependant upon "Many Variables!!--If you were shaped like a bullet`weighed no more than 90Lbs & were laying under the paint, with quality high octane rated fuel, without brakes binding , & no accessories like Panniers & the Engine had at least 16,000K'ms Legit on the clock ,& at sea level ( Not High Altitude & on a "moist day" without Pillion Passenger ,& with Engine having Air Cleaners near new,& providing each Cylinder registered similar Compression Pressure , Perhaps 15 Psi & The Engine had correct Viscosity oil , & Drive train was not "MALADJUSTED " this macine , with small tailwind could be expected to top the Ton!---Now, IF YOU REALLY wanted to know just how good KAWASAKI'S earlier prducts were . locate >1974 S3 --2Cycle Triple Fire Breather & Could out accellerate & out run HONDA K1 750/4 --Reason>?"Power To Weight Ratio"--I hope that has been useful!--Whilst I 'm on that aspect, the Triple sounded to be screaming ( As if Ultra High RPM, Not so, it being Clarke cycle ( 2 Stroke , has "power pulse each time piston climbs to Top Dead Centre & the Four Strokes , Rev so much harder, yet do not quite sound so !!---The Mighty Vincent V Twin was an exception though >Guaranteed each Macine would reach 120 MPH 998 CC British, with very short pushrods, & alongside one at The Ton, years pasyt I noted His Tacho was only showing 4,200RPM!--O K -Cheers, Stay Upright", Des