KX60 won't idle properly

[whatever]

1990 KX60 great shape, runs well. Carburator taken apart and cleaned thoroughly. Air cleaner cleaned and oiled properly. Bike runs well except that after warming up or being ridden it will not hold a good idle. It idles high and then settles down but will eventually stall (within 30 seconds). My thoughts are magneto side crank seal. If this is the case, has anyone replaced this seal and do the cases have to be split to do so?

Thanks, MS