If you can get the Moose cans to hold up there are some larger ones out there. Ive got I think a 16 ounce one from somewhere. If you can Tig weld, the alum tubing isnt that expensive and with some baffling you can get well under 90 db with very little loss from an open header. ~ 200 -300 rpm
The cannisters I made for the BME were 3" dia and 12" long one for each 51cc cylinder. They also worked well on my 46cc Poulan
Adam