The Nor Star Kingfisher is a heavy boat for its size and the positioning of the motor and prop shaft angle certainly matter.
I built one in 1972 powered by a DC Sabre marine diesel (1.5cc) and the engine was in the cockpit which made it heavy at the bow.
I was disappointed by its poor performance in relation to other boats of similar dimensions, in some cases these being powered by 1 cc diesels.
It never crossed my mind that I could have used "ballast" and the boat was sold on 7 months later at a greatly reduced price.
"Fools rush in where experts fear to tread."

Boaty😎

Ideally the centre of the weight needs to be near the middle of the boat.
The original plan looked OK, maybe it required the batteries a little further back. The whole propulsion system did look a bit heavy for the boat.

You could reduce the angle by rotating the motor 180 & using gear or belt drive to the shaft, so the motor is sat over the shaft.

Mornin' Ron,
If you put the motor where the batteries are now - where will the batteries go? They weigh far more than the motor so moving them forward will push the CoG even further forward😲
Re shaft angle; as flat as you can get it for the size of prop you want to use.
This is to reduce the forward rotation moment of the thrust which tends to push the bow down. The further back you fit the motor the larger the angle will inevitably become due to the space needed for the motor and mount!
BTW: Why two NiMh batteries?
I would be tempted to change the set up to a brushless and a single LiPo.
This will significantly reduce the total weight as well👍
You did say the boat is 'a heavy model'! 😉
Doug 😎

12 degrees according to this PDF but I may need to go a bit less say between 8 & 10.

These photos give an idea of where I am going with these modifications. The prop shaft is too steep in this photo...