Hi,
as someone who has owned motorcycles.....I find Harleys as mystical as an old, busted fridge. Back when I was riding, half the bikes you saw busted on the side of the road were Harleys. But this was the Seventies when everyone was buying Honda, and only about 10% of the bikes on the road were Harley.
They eventually improved the reliability while jumping the cost from overpriced to obscene.
If there is one consolation, modifying the exhaust will reduce back pressure. This usally means exhaust will leave the combustion chamber earlier and hotter than the designer intended, which in an efficient engine would ruin it. Still should
damage it, eventually, even on an antique design (it was borrowed originally from a tractor).