How close are you to Ian Hodge? He has a couple of rifles that may work for you.
First is a Stoeger XM1. This is essentially made by a Chinese company called SMK, that are big in the entry level PCP market. The action is a PR900 one, and is used by several manufacturers and just rebranded.
Easy to work on, regulated and comes with a scope and moderator.
Second one is the Gamo GX-40/Phox/Coyote range of rifles. Same BSA made action and cold hammer forged barrel across all 3. Think the GX-40 has the carbine stock. Non-regulated, bolt action uses a BSA made magazine. Bit agricultural, but generally well screwed together.
Both rifles are sub 12 and will do a job at up to 40 yards.
Headshooting squirrels at 50 is a very big ask for any sub 12 though, and more FAC air territory. Personally wouldn't see that as suitable for anything sub-12.