If the leak is more oil than petrol, my guess would be that the guy forgot to put back the oil seal back on the oil pump shaft. This'll mean that the cases fill up with 2-stroke oil and comes outta the overflow pipe. Happened to me. It's an easy fix , just need to order up a new seal, check the Online Part Manual on this site.

The tell tail sign is opening up the gearbox oil top-up plug (???) and see if load of oil spills out, i.e. it's over full.

Good luck