.40 size 2-stroke = .52 - .70 4-stroke
.60 size 2-stroke = .70 - 1.00 4-stroke
.90 size 2-stroke = 1.20+ 4-stroke

My preference it to use an OS 91 Surpass 4-stroke in planes calling for an OS 61 FX. Another good alternative would be the Saito 100 4-stroke. I'm currently doing a review on a World Models Super Chipmunk, 60 size. I am going to use the Saito 100 in it, but I was a bit concerned about the weight of it compared to the OS 61 FX. I was afraid I would have to add weight in the tail to compensate for the heavier engine. But after I looked at the specs for both engines I found that the Saito 100 is actually just a hair lighter than the OS 61. I haven't flown the plane yet, but it should be a nice combo.

Hope this helps

Ken