Suzuki marauder 125cc t reg starts and runs fine but soon as select gear engine turns off

So have you worked on it. Clutch springs could not be engaging and or burnt the clutches. Does the rear wheel turn if you push the bike. The sprocket broke maybe. Snapped the output shaft. Was it working fine and you beating on it before it stopped. Trying rolling the bike and shifting to see if it catches and goes in gear.

Maybe- are the diameter of the fork legs the same and are the fork leg measurements the same centre to centre?

Its possible but hard to answer with out the above information.

check out your side stand safety switch

Check the gear lever isnt stiff or seized.

The most likely cause is the clutch plates have worn out. You have to consider that these 125s are usually ridden by beginners, this means that they will be revved out in ,at least, the first 3 gears.

Check sidestand and clutch lever lockout switches...

Your side stand switch is likely bad.

Your idle circuit jet is clogged, you will need to run seafoam fuel additive in your gas tank and see if this clears it up or clean the carbs.

remove the carb .take it apart clean the jets. and the needle and seat.use small dril bits work well or fine wire..does it got spark

Sounds like a clutch problem to me.
So, with the engine off, can you select all gears? If yes, your gearbox is probably OK.
And now with the engine idling, can you select any gears with out crunching? If this is difficult, then try this next test in an area where movement forward or backwards won't be a problem.
With the engine OFF, foot ON the clutch, the handbrake OFF, and in 1st gear, start the car. BE READY TO TURN IT OFF.
If the car moved forward as you cranked the engine, then your clutch is not adjusted properly or has worn out. That is more-than-likely the smell you got before - burnt clutch disc from it slipping.
You can drive it, but every time you want to start off, you'll have to turn the engine off, select 1st then start the engine. Accelerate slowly, making sure the revs don't rise faster that you're speeding up, and change gear by judging engine revs between gears.
Don't keep driving it as it is, as you WILL have gearbox problems if you don't fix the clutch.