First, I'd use the existing blind nuts to help determine the horizontal and vertical center line of the firewall. This will likely be offset to the left side of the plane to compensate for the right thrust angle. Don't just use the firewall dimensions and use the center of those as a center line since that's may not be how the engine placement is designed. You'll need those center lines as a reference so that you get the gas engine in the right spot to line up with the cowl correctly. Then knock the blind nuts out.

The holes where the blind nuts were can be drilled to a standard size and plugged with hardwood dowel and epoxy.

Can you reach the back side of the firewall pretty easy? Do you have enough access to epoxy a piece of 1/8 or similar plywood? Doing this could help beef up the firewall for the heavier gas engine. You could also sand the front of the firewall down to bare wood and epoxy ply there but make sure that doesn't make the engine stick out too far.

Another precaution might be to pin the firewall. Drill holes through the engine box sides and into the edge of the firewall itself and epoxy some hardwood dowels. This will really help strengthen it.

Some people also like to use fiberglass cloth and epoxy on firewall and engine box joints. It may or may not be necessary on this plane but it probably wouldn't hurt either way.

You might also find some expertise in the gas engine forum on mounting the engine in planes similar to this one.