Hi
Just bought myself a 1978 oddy FL250 and want to install a more powerful Engine
Any ideas what works well
Cheers

Welcome new guy.
In my opinion I would stick with the stock Engine. Reason: The FL250 has no suspension and no "real" frame under it. Also attaching another style Engine will be a butcher style job and will require some engineering and skills. You need a real frame and roll cage to protect yourself. You will need it because with no suspension it is unlikely to handle well and crashes will be common. Give it up.
What's wrong with your Engine ?? Pictures.

I had a 78 and it was a dog no matter what I did to it; though I never installed a comet clutch. Perhaps if you installed one you would be happier with the performance

swapping the Engine is kinda sacrilege on an old ody unless its beat to crap. You can do plenty to soup up your fl250 if you want though. I would start with finding a 1973-9174 cr250 cylinder jug and head. These jugs make somewhere around 28 hp give or take compared to the 17 that the stock fl250 has. The CR250 cylinder and head will bolt right on to the fl250 bottom end. From there put in a cr250 piston for higher compression, buy an aftermarket reed cage intake manifold for the cr250, swap on a 38mm carb such as lectron. make sure your Engine has a CDI (Capacitor Discharge Ignition) ignition not a points ignition. k and n air filter, dg exhaust, and comet 102c clutch are all easy things you can do to soup it up. you can even find a DG head or a watercooled head for it. after all this you will have a machine pushing double the original hp, which will be a hell of a ride without rear shocks