Of the engines choices you have, the OS 25 is the best match. Which model OS do you have, LA, FP, or older Max S ?
Build light, keep it under 30 oz and .25 power is plenty enough..
One of our club members has a porky S1 (32~34oz) and it does fine on LA-25.
With the RSM kit, its easy to get it down in the mid 20's if you dont get carried away with paint.
Use 2.5 or 3.0 ounce tank, 10-4 or 9-5 prop, and .012" x 58' lines.