I too have a Wing B25 on the table. (Actually, it's not on the table 'cause I'm hung up on building operating gear doors, but that's another story!) Obviously, it's not been flown. The current plan is to use a pair of Saito 80's. I'm hoping that's enough power. I've run a couple of different Saito 100's with no problems, and seen several other folks at our field reliably run Saito's of various displacements. (And this is not a Saito commercial, I've also used Magnum 4 strokes.) At one point I was dreaming about using a pair of the (apparently now not available) Saito 90 3 cylinder radials. Then I looked at the price...

I have no direct experience with gas engines, but here's what I think FWIW:

(1) A pair of DLE 20's is probably too much power for your Mitchell. I'm guessing it'll come out around 15/16 lbs, and one DLE 20 would probably be able to handle that much weight.
(2) I watched one guy at our field struggle with one of the small Evo gas engines (I think it was the 10cc model). It's operation was spotty, and eventually he found some kind of problem with scoring on the piston. The engine was replaced under warranty, but the new one never seemed to run reliably. At the same time, the LHS guys had positive experiences with the same Evo engine.
(3) I'd be tempted to look at either the OS 10 or 15 cc gas engines.

Whichever route you choose, I'd make sure that each engine is really well broken in, and has been flown for a couple of hours in a single engine set up to establish reliability. If and when you get to the point that engine failure is pretty far down on you list of worries, then it's probably ready to put in the Mitchell.