EVERYTHING can be fixed on the 5.5 cu. in. singles. I have done it and
so can you.
Try backflow with compressed air or stream of water. That MAY solve
problem. Worst case steel water tube may be clogged with rust. Removing water tube and using it as a pattern to make a copper tube
replacement can be done in an afternoon even if you have never done
it before. Water tubes were sometimes steel and other times copper.
The method is to close the flare and withdraw tube down out of leg.
Install replacement in same way and flare tube when in place.
Best to leave a little extra at both ends of replacement and trim tp
length after getting tube in place.
As long as you have a good crankshaft and piston everything else can
be sourced or fixed without going over a modest amount.