It is only worth the final selling price. at least that is what both parties agree it is worth.
On the open market, $30 for scrap, if you are lucky, or any amount as a non-working engine with a lot missing! Check £&@y and other websites to compare anything similar,,, after a week or so you can see what things are selling for, anyway.
Do you want it "commercially" to do up and sell on? - Or as a project to last a couple of years while you acquire something you'll enjoy restoring? - Either way it's value is whatever it can sell for.
Selling old motorcycles I make a guess, double it and never sell at less than 60% of that value. There is always someone with a price (Money) that makes you happy. Buying, just never pay more than 80% of the asking price... unless it really is what you'll pay full price for, because of rarity or some other reason.
That's all I know.
K2