what exactly has to be done to fit an '89 to '92 Honda RS250 motor to a 1989 MC18 chassis?

Why I ask is, I have an Aprilia RS250 that I am planning to put a Honda RS motor in. I also have an SP bottom end (dry clutch) for an MC18, less crank and clutch hub (was a guys spare parts engine, got it for bugger all...)

I was planning to rebuild and fit the NSR engine, but have figured to look at an RS one instead. If I know whats different, I can use the NSR one as a dummy block.

Or (for those knowledgable) would I be better off getting a '90's TZ engine?
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The Chopster