My B&S 31F707 17hp engine locked up and can't get it to turn, so looking to replace the engine with a B&S 31C707 17.5 hp since it is less than half the price of an exact replacement. Does anyone know if this will work? Or what I need to look for specifically to see if it will fit?

Hi,
The key point is crankshaft dimensions with fuel pump, charging and muffler considerations having some influence. Try the following:
Briggs & Stratton Replacement Vertical Engines

Briggs Engine Search by Small Engine Warehouse

briggs_v

17-21 HP Briggs & Stratton Vertical Shaft Engines - On sale with free shipping from Tulsa Engine Warehouse
And all this info should say a definite "maybe" and probably is more true, but that Type number also comes into play as it tells the "rest of the story."
Peace,
Clarke