Put the engine in its mount.

Grind/snap the points off a 4-40, 6-32, or 8-32 blind nut (whichever one's shank looks like it'll fit in the mount's firewall holes.

Sharpen the end of 4 of whatever bolts fit your blind nuts to a point.

Run suitable nuts down the sharpened bolts a ways, insert bolts through the mount, spin the mod'd blind nuts on from the back and adjust the whole mess so you've just got some points about 1/16-3/32" long sticking up through the blind nuts. Lay the mount on a hard flat surface and adjust the depth of the points to they all make contact at the same time and there's no rocking from one being high. setting the thing on a small square of 1/16 or 3/32 balsa and screwing the points down until they just make contact with the surface will ensure you won't get a false read on the mount being parallel with the firewall. If you can't slip a sheet of notebook paper (about 3/1,000in) between the points and the hard flat surface, that's gonna be close enough.

Fit your cowl over the whole mess and when you're happy with the way thing are lining up, give a hard shove on the engine, and the points on the bolts will dimple the firewall. Those dimples are where you drill.

If the firewall isn't typical soft ARF ply or is coated with something hard like epoxy and doesn't look like it's going to want to dimple easily, just put some masking tape on the firewall in the general vicinity. The tape will dimple easily and locate the drill hole.