Drop the unconstitutional" claim, its a worn out overused political catcall of the times, and IMO in your case, not valid.

The requirement for a muffler to reduce to a minimum, is only achieved by stopping the engine, however the legal expenses to prevail against that requirement in court, ais likely rather expensive,
My lawyer always said he would defend me until my last dollar, so there is some hope

Adding mufflers is the min your should do to resolve, but it seems like your relationship with your neighbor is forever tainted, no matter what you do, sadly.