I use old wet power mounts ll the time. I'm presently flying a Bristol M1a with an Aveox 36/38/3 using an aluminum mount for an OS 15. A little grinding and a hose clamp did the job. You can also make a "clam-shell" mount out of brass or aluminum. I use some left over flashing. If the spacing is too wide you might need to make up a plywood mounting plate that spans the motor mounts and mount your e-motor to it with triangle stock (for alignment) and either a hose clamp or brass strips.

If you need more details let me know.
Walt