The engine mount is always bolted to the firewall, with 4 bolt, and mostly blind nuts. This works fine normally, however some actions are best taken to make sure the bllind nuts work correct.

First, they need to be in the wood. They have these little arms that are pulled into the wood, preventing them from slipping. They must be pulled in all the way, until they're flat against the back of the firewall. Next, drop some thin CA between the nut and the wood, which will prevent them from falling off too easy while installing the bolts.

You could use regular nuts, however that would require you to have physical access to the back of the firewall, which isn't always to case in some planes.

However, since you are going to rework it, maybe a removable hatch is't a bad idea. Mostly the fuel tank is located behind the firewall, so having good access there is helpful in case of tank problems as well. I always build my planes so that I have physical access through a hatch at the room behind the firewall.

Now, as far as I can tell, your engine was loose because it was only fixed with 1 bolt, so I assume the firewall itself is still firmly attached ? Adding some triangular stock at the back never hurts anyway, since you're at it....