My suggestion would be the Magnum 25. It is what Sig recommends. I've flown mine with two different engines, a very old Enya .29 and a Saito .30. Not a lot of difference between the anemic old Enya and the Saito .30 except the Enya was noisy and the Saito is quiet and the Enya went through a lot more fuel leaving much more oil residue on the plane. Both pulled the plane without problem. A friend had one with a hot OS 32 and it was much faster than mine, so it depends on how fast you wish to fly. You can be assured that a .25 will adequately power it for all but someone with a need for speed.

The LT-25 is very light and has plenty of wing area so it flies on the wing and doesn't require going fast.