I have a Clavinova 35/55/65 and 3 keys have stopped working. Did spill a coke there 10-15 years ago but except for the keys sticking a bit everything worked until now.

I couldn't find any tech data for 35/55/65, but it looks to be similar to a large Yamaha I had to repair a couple of months ago for this same issue.

Keyboards can be practically difficult to work in, and can have very fragile interconnects. I wouldn't recommend anyone unfamiliar with working with electronic equipment and the risks involved to attempt this repair.

It may well respond to careful cleaning of the rubberized contact strip that sits under, and is actuated by, the keys. The pcb pads underneath that act as contacts should be cleaned too. It's a good idea to remove any loose dust and debris with a vacuum and a soft brush before lifting the contact strip out, this reduces the likelyhood of introducing problems on other keys. I then used some isopropyl alcohol and a cotton bud on the underside of the strip and the pcb pads. Very little pressure should be used in this process, as extremely fine layers of gold are involved, it's vital for reliable contact. I prefer not to clean areas for keys which were not affected.

There may be an additional issue, if the coke really was the cause then there may also have been acid corrosion of pcb track over the years. This should be repairable though. Inspection with an illuminated magnifier is recommended, or a cheap USB microscope will do a good job too.

Personally, I would say that this really is a job for someone qualified and experienced.