I'm not even sure how this happened as freewheels don't just come apart. I managed bike-shop in Santa Barbara for 10-yrs and have never seen one just fall apart. Usually they seize up with rust.
Note that this is an old freewheel 6-spd design upgraded to hold an extra cog. There's lockring on end of cog-stack that threads onto body to hold extra cog on. I suspect OP tried to remove cog-stack for whatever reason and unscrewed bearing-cap instead of cog-stack lockring. The cogs are matched set due to shift-ramps, so you can't mix & match cogs. And they typically have proprietary splines matched to freewheel-body, so you're not gonna be able to find replacement cogs that fit anyway.
I'm assuming the freewheel body is still attached to hub. It can be removed with Shimano freewheel tool (use QR to hold it lightly). Not sure how it can be put back together, unless you've managed to find and save all the tiny ball bearings. Easiest to remove freewheel body from hub, hold all little bearings in with grease on outer piece, and re-insert body from back side. Then gently spin bearing-cap back on with pin-tool. Remember it's reverse-threaded.
As others mentioned, simplest just to get new freewheel for $12.
https://www.amazon.com/dp/B007Y5GXSM
BTW - chain-whip tool is needed to hold freewheel while unscrewing cog-stack lockring (for cassettes and freewheels).