Hi Cobbadog, I guessed there was some sort of screen in there somewhere, my guess was it was at the bottom of the sump connected to the scavenger pump, very happy to be wrong.
How long are you going to leave the caustic in the cylinder block? Just be careful, if they have plugged any holes or voids with brass or bronze, caustic will eat it very quickly and will also eat the drain tap.
I too am a huge fan of epoxy metal fillers. The best I have ever seen was a product that was a mixture of epoxy resin and titanium dust, we used it in sawmills in areas of severe wear, it was almost indestructible.
When are your parts arriving, I`m looking forward to seeing this running again.
Bob