@btrodden, Brian, Sorry not there to experience what you are when trying to ride the moped, but if revving nicely/engine sounds strong,fluid levels good and not going anywhere under load, then definitely could be a worn clutch issue. If the clutch assy. looks OK on the donor moped by all means give it a try. You can inspect the clutch using link below, Suzuki FA50 Moped service manual, Page 3-6 clutch inspection/test and a lot of other good information that could help. Good luck. I hope this helped you out, if so let me know by pressing the helpful button.
http://projectmopedmanual.info/manuals/suzuki/suzuki-fa50-service-manual-1980.pdf