A simpler option is to ask on here if anyone has an engine to sell. It'll work out much cheaper

O.K. well to be honest, I did reach 100mph on my test ride (priate roads) which is a decent improvement.

The idle on full choke is now 3,500rpm rather than 5,000rpm, and on no choke it's 1,300rpm (near dying) rather than 2,000rpm. I guess that is all related to carburettor setup rather than the source of the problem.

Was suggested to get a smaller sproket so that I would feel a better acceleration rather than top speed.
(I believe I currently have 3-4 teeth more than stock)

I know the pain with carbs, the set currently fitted to my bike are fucked.

The choke circuit plays silly beggers with me, it doesn't close properly, so the idle fluctuates from 1.5k to 2.5k and it is not an air leak and I have the set the mixture screws really lean to combat the choke!

Plus the throttle linkages have loads of free-play in them so I just can't get them balanced. Damn things. But yes the source of problem is the compression. I'm with thunderace on this, find an engine off here, much better than eBay's prices.