Well if u only changed the "engine" not any of the fuelling components which were working previously, I can't figure out what would have changed.

Give more circumstancial information. IE - it was running mint, engine fell to pieces, replaced only the engine and head as a complete unit. I didn't take lots of plus off (well u probably did) I reconnected everything perfectly, maybe u didn't etc - then we can give u more of a suggested fault finding path than "get it checked by a pro" which is what is probably coming!

Cheers

RW