If it's not a "Must Get running this weekend thing"....
I have an Internal flush that has served me well through the years..
Pull it all down and send it all to the machine shop for hot tank, Inspection , Valve job, and to pick up a pack of rings and bearings...and if REALLY gritty..new lifters. While it's out and sitting on the floor..Is the time to do it
Then stop off a the car-wash, and degrease all the shop "Grit" out of the block and parts..and air dry and re~assemble..
Not a BIG fan of additive flushes on super dirty engines..you may get some..but most likely you'll just move some elsewhere where it will hide and someday rear it's ugly head and eat an old bearing alive..just my opinion.tho..