I have a 82 125s. The only engine repair its had ever is an oil filter. still runs well.
for 3years it was thrashed around in fields, then was stolen and trashed. Rebuilt it.
2months later it was torched, all the plastics melted and all the metal covered in thick shit from the roof. Rebuild again and with lots of respray (all done by me i may add for minimum cost).
Then a month ago, stolen again and they chewed the electrics and ****ed the brakes. Sorted it out again.
It now is running fine after some carb adjustments and modifications (smoothing off all the rough edges). Its been running for about 4000 miles, no air filter, 2stroke straight-thru exhaust (sounds ****ing sweet, especially when its running a little lean on engine braking). Top speed on a slight d/hill is 75mph(flat 65) but i don't like to rev this bike to shit as its had a tough life and i haven't taken the restrictor on the exhaust out yet or changed the gearing (1st gear is almost useless)
For me, its a top trail bike. Just the front brake is horrendously crap on the road, on the mud its adequate as i hate front wheel lock up in the gloop.
BTW, are you running it on leaded/lead replacement fuel? If not your valve seats will chew up. Well after thousands of miles. i didnt when it was used off-road for 3years buts still runs good.
Actually, only thing wrong with it is a very small oil leak coming out of the frontside valve cover