The RTV500's chassis won't let a diesel converted to gas engine fit in there-not enough room. The little gas engine that it used on them (twin cylinder) is either a pump or generator engine used overseas, I forget where and what it was used for-but it was an existing engine that Kubota used instead of reinventing the wheel. Sure a little 3 cylinder would be nice but then again, you'd be back to having an RTV900, or if it were a gas burner, maybe an RTV700 (Using a little gas version of a D722?). Then the price goes up. Doesn't seem like it could, given that the 500 engine is already $4500 if you have to replace it.
So back to the RTV500 vs their competition. The Kawasaki Mule 610. The mule is MUCH simpler, rides better, has a better following, has MUCH better brakes, is cheaper, runs exactly the same on a pavement drag race and will KILL the RTV400 and 500 on any hills, uses less fuel, runs quieter, costs less to maintain and operate, costs less to repair, carries a better warranty, air filter location sucks, the list goes on and on. Dealer I worked for carried both and if a customer came in and drove both, they always, 100% of the time, bought the Mule. It got so bad that when we had the 500's, we had to put the Mule's in the back of the showroom or even in the warehouse if we had a bunch of 500's to move, bury them, so that they didn't see them. That was the only way we could sell the RTV500's. And we traded a few RTV500's in for new Mules, but the bad part is the company who bought the traded in RTV500's didn't pay anything for them to speak of because their markets didn't want them. I think about $1500 is the most I ever saw which really surprised me. Kubota really needs to redesign and/or simplify the RTV500, they've done away with the 400 (finally...what a pile), to make it simpler, ride better, more power under loads, cheaper, and improve the warranty from 1 year to 2. It could be a good seller but as it is currently, it pretty much sucks, and the dealer got to where the only way they sold them is if someone came in and actually wanted it specifically-which was kinda rare.