Hydraulic lifters are like miniature oil filters. The passage into the lifter is huge in comparison to the metal dust and sludge in the oil. But the passage for oil leaving the lifter is microscopic. Crap gets in...can't get out.
Options are dirt cheap...if you're prepared to install a stock, used roller cam and used roller lifters. Buy a core roller-cam block, steal the cam, lifters, dogbones, spider, cam thrust plate/bolts, pushrods, and timing set. That's essentially what I did, a few years ago. Pulled the lifters apart ONE AT A TIME for cleaning. Even re-used the timing chain 'n' sprockets.
Maybe you find a broken-crank engine to steal the valvetrain out of for $50. I paid $300, I think...but I used the block, crank, rods, and seven of the pistons, too. I did need different pushrods, as I'm not using self-aligning rockers and stock pushrods will be destroyed by pushrod guideplates.