Hi Toby,
Total loss engines with a mechanical oil pump were a bit of a "half way house" before dry sump lubrication became the norm. The oil "pump" was more like a metering device. Do you have a hand pump as well? If so, at the start of each day drain the sump (or better still, drain the oil at the end of your run, when it's hot and the floaties are mixed into it) and add a pump or two of fresh oil before starting. If it doesn't have a hand pump it was likely designed to just be left as is with old oil in the crankcase. But we're too kind for that - find somewhere to put a few squirts of oil in with your oil can. Somewhere that oils the cams is good, as it often takes a while for the oil mist to get into the timing chest.
With 20 thou piston clearance you won't get much enjoyment out of it, even if you can get it running. If you go down the rebore + new piston path, just leave the oil ring out.
Cheers
Leon