Piaggio "long" case with disk and 13" wheels. Very desireable.
Easy to get parts for as pretty much every 2t 50cc piaggio uses a variant of that engine and parts are a plenty for it. Piaggio engines have a belt that drives the oil pump and water pump that over time grenades and will dust the motor. Replace it for piece of mind or swap to premix. While your in there replace the crank seals.
It can be as slow or as fast as you have the money and desire to make it. 28-30 hp isnt out of the question with some modern kits. A mild 70 kit, pipe and a good tune should see 100kph easy.
Hard to find pieces like bodywork or Derbi specific stuff unless it is used or you get lucky.