I would always prefer a nose-heavy plane, as opposed to tail-heavy. Nose-heavy is so much easier to fix.

That being said, I think your looking at a 160--2.1 2-stroke engine to balance it. I'd lean towards bigger and heavier, if your trying to avoid adding lead to the nose for balance.

I had a ST3000 on mine, and it balanced just a snitch nose-heavy. If memory serves, it only took about 1/2oz of lead in the tail to balance it where I wanted to fly. I could have just moved the battery, but I didn't want to go to the trouble.

I think that if you put a lightweight engine on it, like a Saito 150 or a 120 2-stroke--your going to be tail heavy, and cussing yourself. I'd go for the heavier/bigger engine, and use it later on a 27% plane. A Moki 2.10 will horse that Sig 300XS around like a rag doll, you'll just have to be easy on that left stick. Then, when you crash the Sig or get sick of the way it flies, you can use the big Moki on a larger plane later.