[jdietrich]

I get there's old timers flying their tail wheels from the 1950s that don't have any electrical components

Frustratingly, the FAA hasn't certified a straightforward solution to this problem. Here in the UK, we can use a fully standalone ADS-B transceiver that requires no permanent installation. After a rebate from the CAA, it costs about $500.

https://uavionix.com/general-aviation/skyecho/

[tonyarkles]

I haven't dug too much into the regulations or power requirements around running ADS-B Out purely off a battery but you can get a TSO-certified beacon https://uavionix.com/general-aviation/tailbeaconx/ and pair it with high-capacity LiFePO4 TSO battery https://earthxbatteries.com/product/etx900-tso/ for a not-unreasonable price. I don't know if that would actually be acceptable, but it's a pretty straightforward way to add that functionality to an otherwise steam-gauge-only aircraft.