Flat tappet cams mate to the lifters after a very short period. If you reuse the lifters that were in the engine and keep them on the same lobe they came from, then you could get by with reusing it. .This means marking where they came from at disassembly of the engine. If you didn't do that, thnn trash that cam and get a new one. Rebuilding an engine costs too much in time and money to chance it.

Lifters should have a slight convex shape on the bottom and the cam lobe is not ground perfectly flat. This makes the lifters rotate in the lifter bore and reduces wear.