I've been looking into this quite a bit. Tony Clark makes one for a g-38, and Mick Reeves makes one called the "torquemaster" for the G-62. There is also a site called "Scale Warbird Prop Drives" which is in the process of making a constant-speed variable pitch unit for the DA-50. They have been working on it for a couple of years now, so god knows when it will actually be available, but there is a lot of good info on the site in their FAQ section, discussing how reduction units with scale props will change your takeoff-roll/max airspeed. All of them are pricey. Depending on your project, you might end up having to make your own. Sdp-si.com has a ton of timing pulleys, belts, and bearings if you want to go in that direction.

Hope this helps,

-J