I've just received a copy of the above plan from RCM magazine plan store.

Has anybody built this model?

It gives the weight as 98 ounces and nominates a .60 2 stroke as the power plant.

This plan and info are about 25 years old, and I'm sure I can build a lot lighter than the given figure, and engines have improved a lot in recent years; I figure a .52 FS puts out more power than a .60 2 stroke did 20 years ago.

I'm contemplating a .52 OS FS. Does anybody have any ideas on this?

I haven't built this model, but have built many aircraft from RCM plans over the years. Do not expect a major weight reduction from the stated 98 oz. Balsa hasn't become lighter and wood-building techniques haven't changed that much over the last 25 years.
As for engine I would say a .52 four-stroke engine will be marginal. IMHO a .46-.55 two-stroke or a .70-.82 four-stroke engine are suitable choices for the Gullwing. In 1980 Schneurle-ported engines such as the OS .60 FSR were commonly available and they were quite strong engines even by today's standards.