Hi!
It's very easy to find the center so you can mount your engine properly...The answer is: You don't have to find it!!!

When you have an airplane such as The GEE BEE Z with a round radial engine and cowl you don't mount the glow engine first and then try to mount the engine cowl so that the engine protrudes exactly trough the center of the cowl...

What you do first is to mount the engine cowl so that it sits in a scale position!!!

Then...you remove the cowl. You take the engine with its engine mount mounted to the engine...and put some medium Ca glue on the back of the engine mount. Yes ! You put Ca glue on the mount (be it alu or plastic doesn't matter).

Then you put the GEE BEE Z engine cowl over the engine so that the engine housing protrudes through the hole in the scale cowl. Hold both the cowl and the glow engine and now carefully screw on the GEE BEE Z cowl!

When the cowl is securely tightened to the fuselage... you carefully push the glow engine with its engine mount against the firewall...and hold it there until the Ca glue hardens (use accelerator). Don't forget to check so that the glow engine axle centers in the hole.

Then you remove the cowl and drill the holes through the engine mount into the fire wall. Mount blind nuts and there you are!
That's it!