DUCATI for the Cavalieri brother's sake..
And as much as I love them (the 916-999 are the most beautiful sportsbikes ever made IMHO)
The engines are not exactly great..
it's not like putting a Bugatti engine in it.. more like putting in a Maserati engine
(you essentially need three.. one being repaired, one to move around with and one spare in case that breaks before the first is ready)
So.. in an
endurance race.. I'd put my bet on a 40hp Yamaha if it's facing a 90horse Ducati any day
Also as all XT series engines.. those are the literal work horse.. torque torque and torque
for some mild offroading you can only do better with a higher combustion volume
So IMO a XT550 shifter in a small buggy is a great idea,
non obstructive road speeds, reliable hill climbing power, not too aggressive but snappy enough for good fun.
Yeah.. Personally I'd make a scrambler and keep it for a season or two, but it's not a terrible idea to mount it to a buggy.
GoKart on the other hand is something different (very light, no supsension, one seat, small wheels)
THAT may not be considered stupid but it's not exactly a good idea either..
As I said.. "torque torque and torque".. that engine can pretzel a kart in no time at all.
But apart from it being a dangerous setup:
You'll need new rear tyres every other week or so, it's fun for sure with wheelies and such..
but that will get expensive in the long run.
'sid
you gotta be kidding me if that even a fair deal for 4 predators. And it doesn't have a standard mount. 
