Bike blowing smoke - any ideas?

[ACT_Cross8]

I've bought a 1990 GSX-R1100 and have been tinkering with it to get registered and back on the road. It's almost ready to go, but it's blowing whitish-grey smoke out of both exhausts once it warms up, which I'm guessing will be picked up at inspection.

It sat for 2 years without running before I bought it, so I'm guessing it may have a stuck oil ring or cracked valve stem oil seal or something.

Any other possible causes? How can I go about diagnosing the problem?

Thanks in advance...