Either an SV650 V-twin lump, or maybe the Rotax 450cc 2 stroke flat twin they use on microlights.
Might look a bit like a boxer BMW though...
_________________
MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball