I can't find any replacement exhaust parts online for my '78 620... any suggestions?

Custom, you can do anything you want with it. Don't go larger than a 2.5 inch pipe diameter or low end performance is just gonna go downhill, (I think that 2.5 inch might even be overkill). I run a 1 7/8 inch inner diameter catback straightpipe on a ka24e, and I wouldn't go any smaller. Sure, you could run a 3 inch pipe and get a few lightbulbs (literally) worth of power 100 rpm before redline, but that shouldn't be as important as peak torque, which is around 4000 rpm.

yep... take it to your local trusted exhaust shop... and have them do whatever you want

i have a full custom 2.25" setup with a test pipe and a resonator...