Mariner 3.3hp will be a waste of time on a 6.5m. It'll be like putting the froth on a cappuccino, with plenty bubbles but little substance!
I used to run a Tohatsu 3.5hp (2-stroke) on Zodiac Pro (4.2m) as an auxilliary engine next to a Yamaha main lump. That was a good combination in my opinion.
Plenty of reasons to go down the 2-stroke route, weight and engine width being the over-riding reasons.
Explore the possibility of a fixed external bracket, rather than an adjustable bracket. The Plastimo stainless steel bracket which I had used to rattle like a bag of spanners.
I'd look at the possibility of a Tohatsu or Yamaha 5-8hp. Speaking of which, here's a sweet Yamaha 8hp 2-stroke.
It'll weigh in at 27kg which isn't huge. Also relatively slim so it can sit next to the main engine even at hard lock. I'd look to secure the engine when underway as vibration will be your biggest concern. Ratchet strap to a stainless steel U bolt on the transom knee or deck would work.
Here's the type of brackets I'm referring to.
Fixed Kicker Bracket - 1511 SE - SALTBOSS? MARINE
See what you think. I'm sure you could get something fabricated locally once you've done some measuring. You don't need the depth, but the lip on your transom is your issue. If it was seated close to the tube you should get the clearance you need.
I'd go for it fixed in position. I've seen a few RIBs with the auxilliary located near the rear jockey seat, or in one example in front of the main console. Sounds fine until you have to manhandle 27kg in a rolling sea to the transom without dropping it in the drink!
The other advantage of being in situ on the transom is that you can use it regularly. Trouble with auxilliary engines is they need to be run. I didn't fire up my auxilliary and it was meticulously maintained, but I still found the impeller encrusted in salt at the end of the season. Valuable lesson learnt. It now gets washed down and run in a barrel regardless of whether I used it on the water.
Good luck with your search and keep us posted.