Hi Darren, welcome to Ribnet!
I think the older Zodiacs qwere Hypalon, but I'll leave the Zodiac gurus to confirm that one.
Crack on the floor- how big? Gelcoat (topcoat, flowcoat) can crack if something gets dropped on it. I have repaired dozens of "cracks" on sailing dingies whischj were nothing more than the surface layer. If it;s white, it;s a dawdle with a gelcoat prepair kit (a bit like car body filler type idea). get some really violent grade sand / emery paper, and remove all traces of the crack. Create a well or a groove, again usuing the rough grade paper- this will give the filler something to cling to. Then wash the prepared area with solvent to remove all dust, then apply filler & sand / polish flush when hard. If it;s on the deck, IO guess there may be a rough anti slip surface, youjust have to be creative to blend the repair in when wet.
If your deck isn't white, then same story but you have to mix the colour as well. (or paint the whole lot with anti slip paint!)
The engine theory is a good one. I have a friend who did exactly that, 2 ribs & 3 engines later, he now has a very nice setup. (4m/25 hp - 4m/50Hp - 5.3/50Hp - 5.3/90Hp). Only thing to watch is that if you are planning on upsizing as opposed to just getting a newer boat, make sure you get the biggest engine this hull will handle - otherwise you'll have a lethargic next boat for a while.
Post some pics would be good and help give the ribnet masses a better idea of the repairs.