Missile is still in business. So you should be able the get a conversion kit.
I have never ridden one myself but I know a couple of guys that have them. The steering gets a little "twitchy" because of the steep rake angle of the trike frame. It takes some getting used to. You can correct some of it with custom triples that will increase the "trail" in the steering geometry if you really wanted to. But then again with the 500 powering it the front won't spend that much time on the ground anyway.
YAMAHA 450 HYBRID
85 350X- RED
85 350x -BLACK
86 350x-WHITE (with Goki)
85 250r
83 atc 70
84 atc 70
84 atc 110
09 yfz 450
2006 Arctic Cat Prowler
RZR XP 900