Why cant I go over 50 mph?

Unbolt the exhaust pipe prior to the convertor and see if you gain more power. Does your state have emissions testing? Do you have an oxygen sensor after the convertor? If either question is YES then I don’t recommend to hollow it out, I also don’t recommend an aftermarket part as they normally don’t work properly to pass emission tests. I would not have a problem with a used exhaust system from an original vehicle at the junk yard, I would look at the oil first for normal maintenance from the old owner, engines that burn oil are not good candidates for a used exhaust system.

If you don’t gain power with an open exhaust then a weak fuel pump (low output volume) or bad timing (jumped belt by even 1 tooth) would be the next place to inspect.