Deacon, The older style lawn mower muffler would work good providing that you use in combination with the stock muffler and add it on at the end of the muffler. You do have to make a modification when you use it on a two cycle engine. The lawn mower I have experiance with is the one that has a pipe thread and screws into the exhaust port on the four cycle motor. They come in two sizes one that is used for a engine 5 h.p. or less the second is for 6 h.p. and up. The modification is to drill a hole a little smaller than the pipe at the exit area through the block plate that is made into the muffler. The reason is if you use it with out drilling the hole because it was designed to muffle a four stroke it traps the oil and soon will turn into chared oil residue and clog up the smaller holes that you can't see on the inside baffle area. I used the smaller size and welded it to the exhaust pipe on a Allstate Moped I had when I was a kid. The first time I used it with out drilling the hole out it only lasted about a week. I noticed that it was getting slower each day and discovered it was full of carbon from the oil. I cut it off and replaced it and brazed a pipe fitting that was the same size as the pipe threads on the lawn mower muffler so I could unscrew it to clean out the muffler. Drilled the hole with a hand drill from the exit side. Just used nail for a center punch on the diversion plate and the largest drill bit my dad had, it was about 1/2 in. About every two months or if it started slow down on throttle responce I unscrewed the muffler dropped it in a mason jar of reg. gas and let it soak for about 30 min. (Not very safe the more I think about it kids can do the dumbest things.) I don't think we had spray gumout or carb cleaner then. But a few squirts of carb cleaner or gum out would be a much safer way to get the carbon build up loosened up. I use a flat head screw driver and a small dia. welding rod to help with the hard baked on carbon.
The noise level was no where as mild as the original muffler that came with the Allstate Moped. The original muffler was about 10 inches square with baffles on the inside and it had two small exhaust pipes coming out. The reason I discovered the problem with carbon build up was the original muffler was so clogged up the motor would hardly run. The moped belong to a friend who got it new, he had it for about 1 1/2 years and sold it to me for $45. He sold it because he could not get it running right. The only thing wrong was the carbon build up. If you use the small size muffler as a add on even with the hole drilled you will reduce the muffler noise by at least 25 %. After I get my first bike built I am going to get two of the larger size lawmower mufflers and weld or braze them together with the maximum size hole drilled in them to prevent the carbon build up and start with the header and bend some 3/4 in. hot rolled steel pipe and get rid of the cheese drop pipe that comes with the kit. You can bend the pipe without a problem of kinking the metal with a conduit pipe bender. I found a supply company that has both of the four cycle lawn mower mufflers that are very reasonable prices. The web site is One Stop place for Plumbing, Hardware, Electrical, Automotive, Garden, tools, Paint and Construction supplies.
just go to search and type in "lawn mower muffler" and it will show the round one with a lot of holes drilled and below it is the longer style one that I used. The price listed is $1.79 on sale. I looked to night and they did not have the larger size but when I found the site it was on sale for only $3.99.
It will take me a while to get to the muffler project, I'm in between surgeries and not working up to speed yet. I have one coming up in about 3 weeks and I need a couple weeks to recoup and I will get busy on the motor bike projects. I did get the front motor mount done and if it works as planned it will also have a vibration damper in the mount and you dont have drill a hole. For the muffler project I will post the pictures as I get them done. I also have a line on a variety of small two cycle scooter exhaust pipe that should work with just a bend change at the header where they attached to the exhaust port. It will be the end of May 08 when I open my cycle and scooter shop I have a few commitments for my other businesses before I can open the shop but I will have the cycle, motor bike and scooters and supplies ordered by the first of May. I think that after the exhaust port modification the complete commercial built exhaust system will be less than $60 complete. I have no idea of how loud they will be but it will be easy to add the lawn mower exhaust at the end and it would look like a spark arrester on a stinger type exhaust.