Crank seals gone?

[Ricco1]

My Yamaha RS125 blows out white smoke for the first few miles, then runs clean. I've noticed the gearbox oil level dropping, I've topped it up. Had a read around, seems it might be that one or the other crank seals has gone. The bike had been stood for a couple of years before I bought it, couple of weeks ago. The spark plug is black but not oily. From this description, does it sound like crank seals? If so is it more likely to the the left or right one? Could I tackle this job myself (reasonable tool box and Haynes manual, not much experience)? Otherwise, is it a big job for a garage to do?

Cheers.