Quoted from Newman....

It's a good solid engine that sells for around $120. It's very much like the OS RZ without the tweaks. For what it's worth, this engine was designed by the same engineer that worked for OS for many years and then moved to Thunder Tiger a few years ago. This engine has nearly the same dimensions as the RZ does. The sleeve and piston are the exact same size and design and will actually interchange between the 2 brands. The only real difference is the 21R Pro has a square rod like the RG and the crank isn't relieved, it's the same as the RG crank. This engine doesn't make quite the power that the RZ does but it's really close. But it will smoke an RG on any given day and do it about 30-40 deg cooler. I run them in my Truggy and it rips. Rebuild parts for it are readily available. The p/s set is only $60 and a new rod is about $20. Hard to beat.

He doesn't have a website, (keep's cost's down). The number is 1-800-707-5464. He advertises in RCCA at times. Carries a lot of the most common stuff. Call and ask for the Thunder Tiger Pro .21B (non pull) or the Pro .21B-X (pullstart). '
The B is 119.95 and the B-X is 129.95.