Sorry for the delay here!! Not like me to miss that many days
It should not take long a all to break in. A few runs on the bench as usual with a 9x6 up at rpm and it should be good to go.
8.5x9 is too much prop. 8.8x9 or 8.75x9W is too much too for the SJ-46. 8.5x7.5, 9x6, 9x7 in that range.
Now...
If it is not running properly, check something. Remove the muffler .... look in the exhaust port. Make sure the sleeve is properly aligned in the crankcase.
If it is an original 15 year old SJ-46, then yes, parts are VERY hard to come by. That was the original "rev 1" design. Different crankcase and innerds as todays engines. Usually a new piston can be fit, but depends on what shape the sleeve is in.
My suggestion, first check to make sure everything is lined up. Put it on a test stand, put a 9x6 prop on it (sport 9x6 APC) and run it. Find peak rpm, back off 500 rpm - let me know what sorta RPM you get then.
Bob