Hi Ran, My M15 #400 was bought with the original fixed motor mount and I got a Honda 2 HP outboard - short shaft - in the deal. The short shaft Honda fits the fixed mount but the problem you have is that on a starboard tack, with the engine tilted out of the water, you still drag the prop through the water. So, last year I change to a lift mount to solve that problem. Never heard of a Briggs & Stratton outboard ,...............and 5 HP is vast overkill for an M15. My 2 HP is only operated at about 1/2 throttle to reach hull speed. What is the B&S weight vs the Honda? ............lighter is better. Connie