You really don't need to get into that too much. To fix it so it works with gen bits would be 3-40 quid, to fully build it back up would be around 80, pluss a piston/bore if needed.
My feeling is there is an oil issue coupled to mr rebuild man not being entireely honest about his skills and some buggerd rings.
My engine before i rebuilt it had:
Both crank seals worn,
Both crank bearings worn,
Output shaft bearing and seal worn
Piston rings worn and passing gas
Piston crown melted due to ignition/fuling error from some years ago
Leaky head gasket
Exhaust that was as much weld as metal and leaked spectacularily.
With all that lot she still ran and would give around 45-50mph. Acceleration was hardly shocking and starting was an art but she ran, all it cost me was 90quid to replace all the seals/gaskets and bearings (with gen Yam stuff too)and drop on a good barrel and piston i had.
Do it easily in a weekend if you set your stall out properly.
The best bit, i now know when i'm going to work she will start. I can rely on it as i know its good. Ebay the bike as a non runner and you WILL lose more than it will cost you to fix, and may end up with another dud.
You may even get away with just some rings, i think you would be mad not to at least investigate further. And as for basic technology, take away the electronics and there is nowt special about any engine. Just nuts and bolts.