You can purchase a jett-stream muffler that will bolt right to the ST-51 engine. One piece, will not fall apart, and will make a real animal of the engine. That is available from jettengineering.com

You noted you were looking for a budget solution. That is often difficult... but not impossible....

If I recall, ths ST and COMO 40 and 51 engines had 37mm (or maybe 38 mm) muffler bolt spacing.

This is the same as the Webra 40/50 engines. Also was the same spacing for an older K&B .61 (veco .61) engine. The current Tower .46, Thunder Tiger 40/46, and Tiger Shark .52 are 37mm

You have options. THE ST-51 will run just fine with any of the mufflers listed here.

This info might help in your search.