Welcome!

Chances are good that the screw may have fallen into cylinder but didn't do any real damage. There's a pretty close tolerance between piston & culinder, not sure if that screw coud get in there or not?

I'd first try a long magnetic pickup tool inserted through sparkplug hole. May have to manipulate bike around in various ways to get a catch. However Honda considerately put the exhaust valve on lower side of head, so some possibility the screw blew out, unlikely though.

If you catch the screw there's still that chance that it broke a ring or badly gouged cylinder wall. There's some downloadable manuals for the engine at the companion site, found through the link at top of Home page here. In general a very easy engine to tear down & inspect. Should be able to inspect bore just by pulling head & I think that can be done without pulling whole engine. Needs some gaskets to stick it back together, at least a head gasket & maybe base gasket, o-ring and the cam chain galley gasket.

Several here have commented about their Lifan swaps, a quick search for 'Lifan' should roll out a bunch of hits.