There's OS 52's . The OS 70 is has dimension's closer to a 91

Magnum/ASP/SC 52's and 61's. These are the same on the outside as the OS 52. The Magnum 65 & 71 have dimensions closer to that of the 91.

Saito 50, 56, 72, & 82 The 72 and 82 have a small foot print. The bolt pattern is the same as the 56 but they are 3mm wider between the beams.

The beauty of a four stroke is that you can run it at a reduced throttle setting all day and it will not load up. So you can use a bigger one for when you want a lot of power and the throttle back to fly around slow and easy.

Enjoy,

Jim