G'day BB - welcome to the Forum!
Re your Oceantrek, what has made you decide to change from your Yanmar diesels and surface drives in favour of going to outboard engines?
I am sure that the Yanmars will have a lot more torque than outboard engines of similar power, so you will probably have to install O/B engines of much greater power in order to try to achieve the same speed as before?
Re the ally cat that we built here, here are a few photos of her. She was initially designed for doing kayak tours, with 20 kayaks stowed on the aft deck, and 40 punters further forward.
But they later found that the average punter (usually overweight, and visiting our island for a day on a cruise ship) was too unfit for even a modest amount of kayaking, and all they really wanted to do was to go snorkelling with the turtles (who are real rock stars here) and have a rum punch (or two) afterwards.
So they got rid of the kayaks, installed some seating on the aft deck, and they now carry up to 50 punters on snorkelling tours instead.
Almost 20 years later, I saw a mention of the Kayak Express in the USA which also does kayaking tours.
Kayak Express https://dlba-inc.com/kayak-express/
https://www.moraniron.com/marine-pp
The 70 hp engines were later replaced with a pair of 115 hp motors, and when these were due to be replaced she was fitted with a pair of 150 hp Yamahas - the photo below is rather old, and she now has 150 hp Suzukis.
She initially had a bow ramp for easy beach landings - it has a 'kink' in it, due to being dropped one day accidentally while underway, and it acted like a massive anchor, stopping the boat immediately
After they cancelled the kayak tours, we did some mods at the front and installed a dedicated swimming ladder for the snorkellers instead of the ramp.