A blast from a propane torch or heat gun? The aluminum of the part should expand a bit faster than the steel of the screw, no? You might have to heat while exerting torque on the screws. If you don’t want top apply heat or think it might do damage, then get a block of scrap aluminum (or brass) and clamp it to your piece with no gap, you may be able to whack it with a medium hammer while applying torque to the screw but hitting the block, you won’t gall up the surface of your machine.
Can't do that. The old shaft has neoprene feed rollers and plastic bushings on it. Heat could transfer through the aluminum block that holds it all together. Not safe in this application.
I may revisit your other suggestion, though. Thanks.
The reverse action.
Don’t heat them, but cool them.
Try some compressed air on them to cool them down, shrink them a little and the Al should also open up a little, though the screw threads might tighten between one another, they should widen the tap dia.