RE: 50cc Ultimate
I have not used that engine in this plane.
However I think you may want to approach it a bit differently.
First choose the plane that you want to get, and see if the power plant you are thinking of is in the right weight range.
With larger planes small differences in weight are really not going to matter much.
Don't obsess about where the C.G. is going to come out, as this is something - YOU - determine.
Instead worry about how much total weight you will be adding to the plane with things like a smoke system, etc.
It sounds like the DLE-55 would be a great match for this plane.
With most Gassers I want - BIG - Ignition packs and a large separate electronics battery pack. Both of these add a good bit of weight, but it is your choice as to how you want to handle this.
You can then move your packs and electronics around to put the C.G. at the desired location.
The DLE is not an overly heavy engine, so in all likelyhood you'll end up with the packs forward.
The plane itself is beautifully constructed, but you are ALWAYS advised to go over things carefully.
Typically I apply epoxy on the joint seems of most of my larger planes to strengthen things.
I wick CA in to any surface that will hold a screw, or bolt, etc. to further harden it.
I ALWAYS re-inforce the gear area as much as possible, though I do NOT obsess about having strong gear.
Aluminum gear tends to splay out on harder landings, effectively protecting the fuselage and wings from greater damage... and this can be a good thing!