Let me start by saying I dont know your model of tractor.
The fact that the problem started after the machine was split for clutch work and continues in the face of the replacement of the obvious part makes me think there is a ground problem somewhere.
Does your ground cable run to the engine or the tractor frame?
If it runs to the engine, I would be removing that end from the tractor and making certain it is shiny clean on both the cable and engine.
Reading one volt or slightly less seems odd to me.
If the ground cable from the battery went to the frame, I would be cleaning where it attached and running another cable, with eyes on both ends, from the attachment point to the engine block.
After the engine starts and the glow light is on, if you turn the engine off so the engine stops and then turn the key back to on is the glow light on? If not then I would be suspecting something associated with the charging system has a poor ground, the rectifier/regulator if you have one or even the alternator/dynamo.
A poor ground can create all sorts of weird electrical symptoms often far from where the symptoms occur. A headlamp with a poor ground can illuminate both filaments and make another bulb glow dimly as the power to the headlight finds an incorrect paath to ground.
Dave M7040