I'm thinking a 2500ma 4.8v NiCd for the ignition Battery, with a standard Rx LiFe battery for the Rx and High torque servos.. Both I already have ...

Jack, mount the engine like a regular glow engine. Take the ignition and wrap it in neoprene or foam, that's all it takes.
Mine runs so good it isn't funny. I would recommend a quick break in of a couple of heat / cool cycles, just a few minutes each run with a little extra oil (I think they recommend 20:1) and away you go.

Hi Jack,

Nice to see you make the leap....
With gas power you will need to always consider vibration when installing your equipment.

- use blue loctite on all engine mounting hardware

-when using manual choke the pushrod comes out the front cowl (easiest way) or you can go with a servo to choke it.

-you will need a means to kill the ignition remotely ie; opti kill
  switch (electronic switch between ignition and RX) Thunderbolt
  sells them. This is a major safety consideration. Someday you
  will be Really Happy you have it.....

-make sure engine/oil mix as per manufacturer's instructions
  for break in.

-the cap/plug in the gas tank needs to be gas compatible.
confirm that it is. If not, Dubro  (brown) is gas comp.

-decide if you want a 2 or 3 line fuel system for the tank. If
  you choose 2 you will need a T fitting cut into the carb line for
  fuelling.

- You will need a fuel DOT. (fitting at the fuelling point).

-Separate battery pack for the ignition. Some guys use an IBEC
  which shares RX batts   some guys keep ignition batts
  separate from RX

-RTV silicone for muffler gasket. Fibre gasket promotes the
loosening of the mounting bolts. If loosens, re-tighten when
engine is hot. Use blue loctite as well

A rule of thumb on gas consumption is an ounce a minute. Of course this depends on how savage you get with the throttle.....

Of course there are other things but you will pick this up when flying with others at the field.

Have Fun......

Sorry Jack, yes the RTV silicone replaces the gasket.
Do not use the fibre gasket.

Oyea oyea oyea

Some beefing up with gorilla glue...

finishing Gorilla process now on to Varnish, actually some already applied, but more would be better...

Whenever pre-drilled holes don't line up with your own mount it's a practice for many to epoxy in dowels into unused holes and drill new one's that do line up with your mount.
Unless I'm mistaken, I think that's the point of your question....

Way to go Jack.  Here I am fussing on the sidelines about converting to gas; perhaps I need to take a page from your book and just do it.

I've been reading about the DLE 20; it gets good reviews. 

Question....where do the guns go????

One way to address that Jack is to cut an opening in the firewall large enough to take the engine and mounts. Make a plywood spacer ( hollow ring/hollow rectangle) around the hole thick enough to go inside deep enough to give you the clearance, and put a new fire wall on the BACK of the spacer. When the engine is mounted on this firewall it will stick out less....
don't know if that makes any sense at all....