I was able to mix cogs between Sunrace and Shimano, I believe I put a Shimano 30t UG cog in the biggest position, replacing the Sunrace's 34t Mega-sprocket.

I had to grind flats on the Sunrace retaining ring to twist it off, since the 2 tool notches are well-recessed with apparently no tool available (you could easily make your own from 1/4" flat stock though).

I don't recall if Shimano and Sunrace cogs could interchange on the small-diameter splines, but can on the 3 large-diameter spline positions, at least WRT Shimano cogs fitting on the Sunrace body.

The earlier MegaRange 6 and 7-speed freewheels did not have the large sprockets riveted together. That came later, presumeably to save weight.

Goint to 6, 7 or 8 speed freewheels puts ever more stress on the axle, so a solid axle is recomended as rider weight causes too-frequent axle failure. 8-sp freewheels did not last long in the high-end market for this reason.

I've built hundreds of freewheels, and some were a ridiculous amount of work to put together. Chalk it up to too much coffee and basic stubbornness.

Sunrace quality has improved a bit, I've not had a loosely-assembled one come in over the last a several years. I've seen a few with loosely adjusted bearings, enough to cause a tick-tock rocking sound in certain gears.

I didn't know that corn-cob freewheels were scarce, and would have thought they would still be available cheaply.
Smaller items on Ebay suffer substantial mark-up because of the shipping cost and effort in creating and managing the auction, not to mention Ebay's substantial fees.
Maybe corncob freewheels are becoming like vintage track parts(?), gobbled up by hipsters?