Replacing the muffler mount

You're just trying to remove the barell? Just pull it!

While in fact it's simply a matter of pulling the four fasteners, removing the head and then the cylinder (carb & exhaust too ofc) oftentimes the base gasket is adhered so firmly to the case & cylinder it seems bolted in place - it's not, it's just stuck.

You will no doubt need a new base gasket as it'll get damaged while removing the cylinder - often even w/a new motor that's never been run... but usually you can get the cylinder off with some patience, don't pry at it with a screwdriver as you'll damage a cooling fin - but you can take a piece of wood that fits between the cooling fins to the cylinder itself and tap it a bit, one side than the other until it works free. Sometimes a bit of carb cleaner on the exposed bits of base gasket & left to soak will help... but it's mostly patience lol

Here's a good tut on a top end rebuild for more info & pics: http://motorbicycling.com/f39/motorized-bicycle-engine-top-end-rebuild-297.html