A10 engine won't fit directly - crankcases are different, as are all the external parts in one way or another. A10 top end will fit on FH bottom end but later 'big fin' A10 barrels/heads have a diferent exhaust port angle (earliy A10 barrels/heads are similer to FH but I'm not sure about the exhausts). Anyway decent A10 stuff isn't cheap and going back to a twin will need a lot of different bits and pieces (engine plates, studs/spacers, mag + dynamo, chaincase inner etc).
I'd either go mad and stick a Triumph engine in it or convert to a 350......
(My current 500 single started as a 650, after blowing the original engine up I converted it to a single and got rid of absolutely all traces of the twin in disgust. I've lately been thinking about returning it to original spec and I'm regretting being quite so ruthless!)