Best way to flush engine after rocker arm failure?

Flush, Flush and Flush again.

I have used diesel to flush out engines in the past. Don't know the good or bad of it (seals etc). but you can put a lot of flow through the parts of the oil drain back system to flush them. Valve covers on and drain plug out and fill the valve cover fast so they kind of fill up.

As for the supply side???

I have heard of old timers running the engine (no load) with diesel in to help flush things out.

Good luck