Nothing that sophisticated ...Since this rifle was produced as a mag fed bolt action, what does the cartridge rest on inside the action before you feed it forward with the bolt? Is the stock serving as the cartridge "lifter"?
I have a Marlin 101 single shot bolt action, so I'm just trying to visualize how you are single loading the ammo without the cartridge falling down and out of alignment with with bolt and chamber.
suitably shaped balsa block wood, instead of the lifter_...what does the cartridge rest on inside the action before you feed it forward with the bolt?...
pine wood would be the most suitable because it is also excellent for obtaining the plaster mold in which to melt the lead...My mom's model 75 had a piece of pine? in the bottom of the action intended to make it a single shot. Worked well. Would have worked better if more care had been taken in it's construction.
I had one project working with a youth rifle - Marlin 15 YN ... It had to be turned back into a Youth Rifle ...Wow, you are quite the craftsman ! Excellent McGivering. Look on the bright side, you have a good bore, and the stock is genuine walnut.
When I saw the rifle described, before you started work, my first thought was.... Make a kids single shot 22 by shortening the buttstock, and forend, Cut the barrel back and reset the front sight.