In the absence of other replies....
Just wondering which boat you've got - you described it as a catamaran rib designed for twin motors but looking at Aquarius website https://www.aquariusinflatables.co.z...ll-semi-rigid/
they seem to do two models - a 5.5 conventional hull designed for twin motors and a catamaran (like a zapcat) designed for a single.
I'm guessing you've got the 5.5 monohull linked above.
That being the case, the boat was clearly designed for twins and the website doesn't show a single as an option - which is odd.
It's rated for up to 115hp so you should be good to that, even moving to 4str - just check the weights of your intended motor Vs a pair of 1990s vintage 50hp and I'm sure you'll be well in.
Given the motors were designed to be very close-mounted (not much space to do differently on a 5.5) the transom structure is probably fine for a centrally mounted single but if in any doubt, get a local boatbuilder or surveyor to have a look.
In the UK at least it would be pretty unusual for a boat of this size to have twin motors - the extra cost (purchase and running) isn't really justified unless doing serious safety / rescue work. Your do of course get a benefit in terms of redundancy / reliability but this is somewhat eroded if your don't have separate fuel & electrical systems - again, unusual on a small rib (dont know if you have these). Also it is very hard (IME) to get smaller outboards (below about 75hp) with a contra-rotating gearbox. Having a pair of "right handed" engines will make the boat a pig to drive Vs a single. If you're getting quotes for twins, make sure the supplier bis quoting you for a pair -i.e. one conventional "right hand" rotation and one contra / "left hand" rotation.
If it was me I think I'd find a nice secondhand 100-115hp, pop it on and go have fun