How easy is it to swap/change/fix gears in a motorcycle?

I'm looking at a 1990 Suzuki Katana that has 13,000 original miles, but the 3rd gear is stripped out and does not work. The guy selling the bike already has a spare engine for parts. How easy is it and how long would it take to fix the problem?How easy is it to swap/change/fix gears in a motorcycle?
Is the rest of the bike in very good condition and/or is the selling price cheap ? Is the spare engine OK, if it is the easiest fix is to just change out the engine. If the spare engine is no good you could still probably get the needed gears out of it. In order to repair the tranny you will have to pull the engine, remove left+right engine covers, clutch and ignition plate to access any thing that holds the cases together. There will probably be some case bolts on top and bottom of the cases find them ALL. You do not have to pull the head or cylinder. Probably what happened was that the bike has been crashed and fell on the shifter which forced the shift and bent the shift fork, consquently, two or more gears have now become damaged from slipping by. It is a big job and probably not for the home mechanic, but hey, by a manual and try to repair it as a winter project.How easy is it to swap/change/fix gears in a motorcycle?
It is a major job, you will have to split the case. I do most of my own work on everything I own and it would be a job I would take to the shop to have done. He is selling the bike for this reason, do not buy this bike, it will cost more to fix it than it is really worth.
do your self a favor buy something else,more than likely the bike was beat to lose 3rd.there may be other problems you DONT know about.keep looking there everywhere.