well... hmmm, that's a problem. i suspect, and as mentioned, there's no way to tell for sure without what may turn out to be a failed and possibly expensive experiment, whether or not a 700c fork and a 650c rear wheel, or a 650c fork and two 650c wheels, is the better option.

if i had to guess i'd say matching 650c wheels. after all, the frame may be designed for 650c, seeing as how the rear triangle is obviously a 650c rear triangle. but it's just a guess. i hope the frame was really, really cheap.

to facilitate a 650c/650c test, i might look into borrowing set of old mtb wheels. they are readily available, can be cheap, tires can be had down to about 25mms, and a 650c fork will usually accommodate them. you may need a long reach caliper brake. i ran my 650c road bike for years on a nice set of mtb wheels. good luck to you.

i googled that frame and it looks like a nice one, it would be sad if it turned out to be unusable.