I've read in other threads here, and am waiting to try myself, that some use those lazer levelers sold at home centers and or the cheap laser pointers may also do.

Strap your plane down as noted above.
Install your motor and set the laser to point at the center of your motor.
remove the motor and install the cowel and the laser will point at the location to start your trimming.

this also works with trimming for the valve covers and such.

Hope this helps also

Bill