I don't frequent these parts much anymore; well done on acquiring a very tough, seaworthy craft. Although I owned a twin set-up I helmed many single installations in SA. The builder, Malan Conradie is a personal school and army friend of mine. I am sure he would give you all the advice you need.
First off, no-one would believe this story; we found anyone sitting on the aft bench seat always ended up with an unexplained wet back. It was becoming so ridiculous that I had my wife helm the boat whilst I hung over the transom to see how it was possible for water to effectively squirt backwards against the flow of everything. When the boat hit roughly 24 knots it all became apparent. The position of the central bung was the cause of the problem. By simply rotating it so that the ‘butterfly’ end is vertical you will have solved the problem. What happens is when going along the force of the water is so great on the edge of the bung fastener the water is propelled over the transom back into the boat – weird huh?
Your next problem is the way the engine is mounted as well as the positioning of ballast with a single engine installation. If you could draw an imaginary line between the bottom inside corners of the hulls, the cavitation plate of the engine must be in line with this. We found by placing a bag or two of sand in the forward anchor locker places the boat more squarely in the water when landing. The boat is more stable and you will find driving her faster than you ordinarily would over choppy water affords a much more comfortable ride as she gets up higher on the foil.
Good luck; let me know how you get on.