@Lonny I don't know if it's been mentioned already but a Hyabusa engine would be incredible in these.
All the HP and high RPMs to make anyone happy, lightweight, parts / community support from more than enough vendors, price is right, electronic shift a possibility. No donor, just engine/transmission/ecu and figure out wiring through a standalone or kit = muchhhh cheaper and less fuss.
I do think this would make the Goblin a 'motorcycle' though which could be more difficult to go through registration / insure (or easier? I dunno)