I have no idea of what the limit of distruction on a bicycle engine is. I am quite sure you could find a engine that would turn it into a meatball of metal... However if you want to see a really neat engine "I have no idea if it would fit and it goes WAY Beyond legal" check out the Lombardini Diesel's they have some single cylinder diesels... however it may make your bike into little meatballs Not to mention I have no idea on cost.
Ahh just noticed your in Western Aus you could always slam a Victa on there
Those things are incredible for power and strength/durable
i dont think my dad would be too happy if he found his lawnmower in bits, trust me!
um i think ill stick to the old chainsaw engine, since my dad has a spare one he never uses, and the chain is broken on it so i dont think he will want it.
anyway thz for the work