Here's what I did to mine....don't read it as the Gospel, however...

Should I reinforce the firewall?
I did. Drill 3 or 4 holes in each side of the firewall and epoxy in toothpicks.

f I don't use the stock tank(I am using a low profile sullivan 11 oz.) what is the best way to seal the hole in the firewall around the fuel lines?
I used the stock tank. Just seal the fuel line holes with silicon.

Should I go ahead and purchase the nylon motor mount?
I did. Used a Dave Brown, but think the one they supply is fine.

Did anyone cross the cable guides going back to the rudder and elevator to create less friction for the metal p
I didn't, left them stock.

The only other things I did was to pin the control surface hinges (toothpicks, again) and seal all the control surface gaps.

I know there are some other mods but mine flies fine with an Enya 50CX.

I think you'll like this plane and it will fly just fine with no mods, I'm sure.