RE: SIG 4*120 ARF
I have two Sig 4* 120 arfs ( and have had one that has crashed. Another was bought as a source for parts for the flyer.). I love them . They are wondeful planes. Fly like a dream.
I used to have a OS 120 four stroke in it .But when I put a saito 120 FS in it it and it came alive. IMHO the planes needs and can easily handle the extra power.
I have 2 DLE 20 and two DLE 30s. The 20 is less powerful that my OS 120 FS.
An OS 90 four stroke is roughly the sme power out put as a 2 stroke 60 and so the plane would be severly underpowered. It would fly , but not be as interesting and satistfing.
I think would also be a step backward in performance.
The installation should be no problem . The 20 is a beam mount engine and it should fit right nicely.
Watch the balance very carefully and make sure to reinforce the firewall well .
The arf version is not available any longer . I wish sig would sell it again and come out with a gas version.
I will try to answer any additional question if I can.