The EGR is fine not being hooked up. It’s just a plug at that point.
as for the engine run on. AKA. Dieseling. You need to preform a lean drop on the carb. There is a Weber manual in the manual section in the sticky area under the factory first gen manual.
As for slowing loosing power get a new coil. As they go bad. They have a hard time in the higher RPM ranges.
oil pressure can be the pressure relief spring is good bad or it’s a pump problem.
OR if it had the balance shaft removed. It might be he didn’t cover the holes properly when putting the bearings in that come with the delete kit. Or if it still has them they are wiped out and should be removed.