When I repowered my Bobcat, I used these guys as a starting point:
http://www.smallenginewarehouse.com/RepowerItems.asp?Brand=Case&Model=1816
Once I knew what engine would fit, I substituted the engine accordingly and then built my own conversion kit.
I don't know about the Predator 420, seems a little small for this application. As you can see, Small Engine Warehouse is squeezing in a 18 hp B&S V twin, so space doesn't seem to be an issue.
Based on my experience, going up in horsepower (in my case, 16.5 to 20 hp) is a good thing.
Here is another place that you may want to look:
http://www.carrollstream.com/default.asp
Hope this helps,
SR