[HawgRyder]

The most critical time for an engine is when it is first started.
All the assembly lube gets flushed into the sump...the oil has to feed into lots of places that might take a few seconds to reach.
Constant RPM is not the best (might not do much harm) but I always try to vary the RPM from just above idle to maybe 3000 - 3500 RPM just to be safe.
With a water cooled engine...I run it until the thermostat opens and the top rad hose gets hot.
With an air-cooled...perhaps the oil line to the front cooler or something along that line might be the best indicator.
In any case 20 minutes sounds about right.
Then....I change the oil AND filter!!
All the stuff that you missed when doing the rebuild will be out of the engine (and we all miss something...LOL).
This makes for the best chance of a long term engine in my opinion.
As to your engine...you can check the cam surfaces by taking off the top valve covers...if the cam looks OK...you might be lucky.
Bob