The best option at the minute is the Tyga 300cc kit, which will give you 75hp with no reliability issues. It's also the same weight as the NSR, which is where just about any other engine will loose out.
The modern 600's are very compact and light, but are still heaps of pig iron compared to the two stroke, simply because of the extra parts involved in a 4 stroke engine.
How big did you want to go? Maybe you'd be better with an FZR400 frame fitted with the 1000cc engine?
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