Haha I think I might have to go and ask my local mower shop if they know where any are!
Go new crank seals and rings for sure, I remember reading somewhere that one piston has a bigger dome (so more compression) so it might be worth getting another one of those. That might be BS though. Doing new bearings and stuff on one of those would be easy as too, they've got a normal crank instead of a stupid press fit one.
I know the coils are piss weak on them, you could do twin points and twin motorbike coils but you'd need to either run a total loss battery of make a generator setup, it's a lot of messing around. You might not have problems with the coil though.
I'd leave the ports as they are hay, it might perform worse and you're kinda stuffed if that happens. Clean them up though, Victa castings are generally pretty rough, these engines might be different though.
Go for a bike carby for sure, no Victa carb will be good enough for performance use on one of these, they all have tiny venturis and very limited adjust-ability. It'd be pretty difficult to fit a metal victa carb to a twin anyway. From what I've seen (just parts diagrams) the reed valve set up is nothing special so it might be good to do something about that too.
If you wana get some specs I can design an expansion chamber for you.
Thanks
Hayden