I didn't know the 512 came in .22mag; thought it was only .22lr. I'll have to look into that. That 80% receiver sounds like a fun project, too. A little pricey for what it is, tho...Have you thought about a CZ-512 semi-automatic in .22 magnum? They are sweet little guns.
How handy are you? Want a project?
Here is a link to a company that will sell you most of the needed parts to built you own. Bad news it requires some effort to finish the 80% receiver and put it together, good news is you don't have to use a FFL, no serial number (unless you want to sell it), and it's truly a custom build to your own specifications.
https://ruger1022receiver.3dcartstores.com/Kit-22-WMR-MAGNUM-Receiver-Anodized-Black-_p_41.html