Making an engine cradle probably isn't terribly hard, but considering a big block and small block industrial engine mounts can be bought or made easier there is that. The other half is that the bike engine will have a specific chain and sprocket size, like a 428 chain or 520, where as an industrial engine you can pick and choose which chain and sprockets you want much easier. Which also means adjusting your final gear ratio will be a lot easier than say trying to locate a larger or smaller sprocket for 428 chain.
Haven't looked a chain pitch chart in a few months so I don't know how close 420 and 428 chain are in relation to using each other universally (doubt it). Just remember the faster you go, the better the brakes have to be. 35mph geared for a 40 series on a 420 Predator would be at that engines top end depending on how it was geared and tires, where as a bike engine, that could be say top end of 2nd and bottom end of 3rd gear, with an extra gear or two after that to work with.
Gearing a bike engine gives you more overall flexibility in that you have gears to work with for speed. So you could gear it deep and still have decent speed and awesome take off, but can't really be said for an industrial engine.
