If the motor came with it's little Metal X mount then hopefully, as it's a similar level as an FT standard, it should fit right on. Otherwise, you might be able to make the firewall fit the motor by drilling some holes - hopefully the holes that are already on the Firewall don't make it so you would be drilling new holes too close to them.
Otherwise, it won't be hard to cut a new Firewall for yourself from some 2mm or 3mm plywood - which is what I've always done, though it can sometimes take a couple of attempts to get the holes in the right places.