Birdog,

I used the included engine mount for my 1.20FS however I have been faced with a similar problem before.

What I did was to bend an L shape on the end of a piece of music wire, slip it through the bolt holes, far back enough to be able to hook the blind nuts onto the end of the wire. Then carefully pull the wire back through the firewall till the blind nuts seat in the holes you drilled.
You can then tighten them into the firewall by inserting the motor mount bolts and tighten them.

Hope this helps

Ari