Hi Quinnbrian. I run a Lister ST2 which is very similar to yours. I found that the cheapest way to silence the exhaust was to build an underground chamber with a long underground pipe to act as a muffler. You will need a shovel, one cheap plastic dustbin, one length of 3" corrugated plastic pipe and about a half a ton of concrete.
Dig a round hole 200 mm larger than the diameter of the dustbin and about 100 mm deeper.
Pour about 100 mm of concrete in the bottom of the round hole. drop your dustbin into the hole and press into the concrete. Fill the dustbin with water and ensure the bin is somewhere near level. Before filling around the dustbin with concrete place a 100 mm piece of 100 mm plastic pipe at the bottom of the dustbin. Now fill around the bin and over the top of the pipe with concrete.
Allow to set for at least 24 hours. Using a grinder/stanley knife cut the plastic bin into bits and pull it out. you should be left with a very nice cast concrete chamber. Where the 100 mm pipe was buried, knock out any concrete that has filled the pipe.
Dig a 150 mm wide trench from that pipe to wherever you want your exhaust to vent, if you can run it down hill all the better. pour 100mm depth of concrete into the trench and press the 3" corrugated pipe into it. Cover with another 100 mm of concrete before back filling trench.
All that is left to do is to shutter the top of the chamber, drop your exhaust pipe into it and concrete the top.
In my situation, being on a hill, I ran six meters of corrugated plastic pipe, set to fall, to avoid the expansion chamber flooding. I also installed a fly mesh on the outlet to keep out the ants.
Total silence from the exhaust, doesn`t however kill the intake or mechanical noise.
Just in case you are concerned about the corrugated pipe melting and causing problems, it never has and even if it does it was only a shutter for the concrete which should last for centuries.
Bob