Great bikes. Fantastic motor, great handling, pretty good everything else.
Suspension wise, they are entirely rideable in stock condition, particularly if you weigh 60~80kg.
I'd consider a full service to be essential. The shock is very easy to rebuild - any suspension specialist ought to be able to do it with their eyes closed.
How far you go from there depends on how serious you are: heavier riders will benefit from stiffer springs, the valve piston in the shock is primitive, all damper-rod forks can be improved with cartridge emulators.
The forks have less travel than advertised. IIRC, the forks actually have more like 9" of travel, rather than the advertised ~10".
Personally, if I owned one, I'd be looking at fitting an 82-84 KX front end for the extra travel, bigger tubes and disc brake. Then I'd fit cartridge emulators and springs in the 0.45 range.
I'd also look at fitting a shock with a 46mm ID and a Gold Valve. Resprung to my weight.
Depending on how it handled with ~12" of travel, I would not be against bringing the travel back down to 10.5" or so.