I know the FR model is the air floor version, but double-check the specific model. The MkIIC uses a short shaft (15") but the MkII (4.1m) uses a long shaft (20"). More details on Zodiac's website: Zodiac Marine
These are the new Zodiac Futuras that have the oversized Tubes. Could be you've got an earlier version? If you use the wrong shaft size, the boat will struggle to plane, or cavitate, etc.
Regards engines, then ideally I'd look for something a little bigger, possibly 8hp. 3-4 adults is a big ask for 5hp, fine if it was just the two of you on board. Also, if you go 4-stroke, then the engine has to be transported a particular way to avoid oil leaking into the cylinders.
Main engine 25-30hp is fine if it's being transported on the transom and trailered. You can use a transom saving device to support the outboard skeg etc.