you could drill a hole into the rear section of the muffler, tap it and purchase a fuel nipple from your LHS and seal it with some PTFE, i had to do this on one of my Truck mufflers long ago

your engine may be designed to run WITHOUT a pressure feed and just an open vent instead, does it come with a manual or can you find an pictures of an identical engine with the original muffler? (if so, does that muffler have a rpessure nipple on?)