Engine differance
 

I have a .61 2 stroke engine what size would I have to go to in a 4 stroke

.70-.90ish depending on what you want.

Agreed.

the general rule is that for the size of 2 stroke you want to replace with a 4 stroke, the next general size range up in 4 stroke engine will deliver similar power as the replaced 2 stroke.

It also depends on the brand you choose. Saito and the current OS Alpha series will make more power than a Magnum or any of the older OS 4 strokes. A Magnum .91 4 stroke will probably match the power of your .61 (although the way it accelerates and the general flying characteristics will be totally different) while the aforementioned top end engines will probably outperform it.

Another way to look at it is the weight of the engine you want to replace. Also ground clearance and space for installation.

The 61s of 30 years ago usually weighed in the range of about 18 to 21 oz w muffler. The OS 70 to 81 alpha, Saito 82, 91, or 100, or YS 70 to 80, or equivalent all fit that range; power will range from a little less than the old 61 to much more. They are longer than 2 two strokes which may caused installation problems. They turn larger props, which sometimes cause ground clearance problems, though you can usually fix that by increasing pitch to keep down the diameter, or with 3 bladed props.

More recent 61s weigh more like 24 to 26 oz and generally make more power than older 61s. For those engines, you definitely want at least a 91 4-stroke to get equivalent power, or any YS But you could also go with much more powerful 4-strokes without a weight penalty, like the OS 110 alpha (discontinued?), Saito 125, YS 110-115 which are all very strong engines. Then prop clearance and installation might limit you.

Jim