I wonder if they did'nt install the firewall backwards, as there is usually scribe lines for the engine mount.

I would suspend the plane vertically on a ladder so the firewall is flat, then you can position the engine to get it lined up with the cowl. You will probably have to lay the engine inverted in order to slide the cowl over it for positioning. The centerline will be offset a little to allow for the right engine thrust.

Cut the cowl after the engine is mounted.

I have a pumped O.S. 91 4 stroke running inverted, runs just fine. put a remote glow igniter on it for easy access.