Oil seals last a long time and even 30 years ago they didn't get brittle. On a two stroke motor like most mopeds or snow-mobiles etc the proper test is to make seals (plates and gaskets etc) to plug the intake and exhaust holes.) then put the piston at the bottom and apply not more than 15 psi of air pressure in the spark plug hole. Sounds like a lot of bother to me and I have been a motorcycle mechanic for 40 years on BSA NORTON, H.D. Yamaha and mobylette. If the main bearings are bad then even new seals will not seal. But if seals are leaking then you should be able to see the leak. On any two stroke motor Chainsaw etc a leak will make the motor hard to start and it will not idle good because it will suck air in through the seal easier than through the carb and will not get enough fuel (gas) to operate. This is called a lean mixture and it is simply not enough gas for the air. When you do get the motor running it will race too fast but maybe die as you open the throttle . A rare case is where the motor backfired and the excess pressure blew the oil seal part way out but maybe sucked it part way back in, it still leaks.