You can chop and see if the plug looks golden brown. If it is and runs ok, I would keep it the same. If it looks light or blistered, a step or two cooler is worth trying.
The higher power versions of a given type of moped engine generally use colder plugs, all else being the same. The 2HP for my Batavus uses a W240 whereas the 1 HP uses a W175 (essentially same engine, but bigger carb on the 2HP). Generally, a colder plug is used if you run the bike open throttle most of the time.
The Casa looks like a neat bike.