Good luck finding an engine. I managed after over a year of going through the auction sites to acquire an overbored 250 as others in this topic have mentioned. I supposed it was that or nothing.
I was lucky in that all the work and research was done by the previous owner, or one before him. I simply lowered the engine and swapped over the barrel, piston and head. I have not changed the fuelling, nor ignition. And I've been running it for a while now, looking at my records since it was March last year, but I've only put on 1,100 miles on it. (I'm slacking now that my daily rider is so very much more reliable than it used to be - 10k on that this year).
As far as i'm aware it seems to be going well, and certainly suprises some cars on the traffic light race, occasionally.
When I take the opportunity to ride it, I do love it! It's just a shame the gearbox is so sloppy
As far as I can gather, having a 300 over the 250 is indeed an improvement, I could instantly tell as much when I first installed mine, You could almost allow it to propel itself in first by just slipping the clutch. But the extra what 1.5/2hp? experience soon fades and you wonder was it really all that worth it? One of the big reasons I upgraded was my original had a poorly scored piston, and considerable piston slap. But otherwise I might not have bothered, I may have probably found no difference at all if my original 250 barrel and piston were of good spec..
The fact I get considerably more MPG on average per fill up just proves how inefficient the old piston/barrel was, as the 300cc is supposed to be a bit more thirsty in practice. I always had low to mid/high 50's, but now get a good constant mid 60's just about every time.