With right thrust, you cannot mount the engine on the center line. I take a yard stick and tape to the center line of the fuse, hold the engine and mount in place and mark where the prop hub will be centered, eyeballing from the top. Have never done this for up or down thrust but most plans come with a drawing of where the thrust line is, the same theory could work.

Drill the mount holes appropriately.

Mount the cowl after the engine is mounted. Remember to measure the distance from the from of the cowl from the firewall and ensure spacers are taken into consideration if needed for mounting the engine mount.

Ed

PS for 140-120 4 strokes I figure 1/4" round about from center for right thrust, and 3w100-150 3/8" from center if you just want close.