Where can I get a 20" 1" threaded fork with the mounting hardware for a disc brake?
Eric, that hub does not look like it has the required mountings for a disc to be attached, as there are no bolt holes and no threaded boss to screw an adapter onto.
So perhaps it's best to forget about a disc brake conversion.
The rim appears to have been laced slightly offset.
You should be able to correct this offset with some careful use of a spoke key:
If you start at the valve and then slacken all the spokes on the right by unscrewing the nipple half a turn and then tighten all the spokes on the left side by tightening the nipple half a turn, and then see how it looks. As long as each nipple is moved the same amount, this should ensure the wheel stays true.
You may need to repeat this process to get the rim centred correctly within the forks.
Check out this link for further information regarding tensioning the spokes and truing the wheelIf the wheels were correctly trued using a suitable wheel truing jig, then problems like this should not arise!
Alan