Driving it some will not create any more problems than you already have.
I suggest pulling the plugs again and see how they compare to each other and your memory from previous observations. If #5 is oil fouled again and the rest look okay doing a leak down test may help determine if the problem is with the piston rings (valve guide issues will not be obvious).
I do not recall if you have previously determined the origin of the engine (year and any rebuild information). The engine I pulled from mine had a rebuild tag that identified the rebuild information such as bearing sizes etc. as well as its origin being from a car (do not recall the year off hand).
