You should be able to get a new muffler for around $ 50 a any one of 15,000 mower shops in the US.
adding post & custome should still come out at less than $ 100.
I buy a lof of stuff from Jacks <http://www.jackssmallengines.com> not the cheapest but excellent service particularly as I am getting them sent to OZ.
While no muffler baffels might not hurt your engine, they also could lead to insufficient back pressure ( same as fitting flow through mufflers on your car ) which may cause the engine to run lean which may cause it to overheat .
The real rough & ready test is after the engine has been running for about 1/2 hours so it is nice & hot turn the choke on slightly.
If the motor starts to 8 stroke & blow smoke then all is fairly well with the noisy muffler.
If OTOH the engine runs beter with a little ( and I meal little ) bit of choke then that is a sure sing of a lean running motor.