The 55 AX with pipe is enough. I would not fit a 65 AX with a pipe as that would be too much, but I can see the attraction with simply bolting in a 65 AX and stock muffler for simplicity. I have a 65AX as well and it's very light. But will the stock muffler clear the nose gear?
I'm using the PIpe combo from
www.justengines.co.uk with the wide header. I cut off about 3,5 cm off the header to work with an 11x9 APC.