If you are building an engine today (~2023), Professionals are not building Hydraulic FLat Tappet camshaft engines? Don't try?

That is out of a BBC, Gen7 engine, 604 cid. This was in what's called a "gen-set", which is a stationary power plant. That would run day and night for years at near 2000 rpm. It had 1.3 million hours on it, which is about the same as 780000 miles.