The flooding hulls were only fitted in the early days of the XR18, when they didn't prove popular the mould was altered and a larger planning pad added. I can't remember the flooding hull having breather holes but you could have a chat with Gary at:
He worked for Osprey at the time and built the second mould. I would use a nice piece of stainless steel plate screwed over the hole to block it off, but add a good size bung so you can drain any water that does find its way inside.
Have you seen this:
I have the original sales brouchure on my desk at work so can look up the engine options but it will be Tuesday. They should go well with 90 hp, however we have seen one that had a 100 v4 suzuki that had extra rear seats and rode very bows up, may have been because the owner didn't know how to trim a boat.