[Kube]

I use a unit that is very similar to a brake bleeding unit. I attach where the oil sending unit is mounted. This will push oil though all of the bearings, etc.

I started doing this to every engine I've rebuilt since the 1970's. Some with hydraulic lifters... this will push oil through them, the push rods, etc. Not a concern with a flatty but just an example of how well it works.
I am confident you could buy one of these quite reasonably these days at Harbor Freight, etc.