So after some searching it seems like the answer is these are the plugs Sena uses:
https://www.amazon.com/gp/product/B013JRWCBU/

I'm buying a few bits and pieces I can try building my own connector with. We'll see how it goes! :D

You can change the ends out without soldering. You need these

Letool 30 Pairs JST 1.25mm 2 Pin... https://www.amazon.com/dp/B013JRWCBU?ref=ppx_pop_mob_ap_share

And these

UY Wire to Wire Connector K2,SINCODA 100PCS Waterproof Gel-Filled Orange Clear Button Telephone Wire Connectors UY2 Butt Splice Connector K2 Network Cable Terminals https://a.co/d/a7w34cq

This battery is the correct physical size with the wrong plug - https://www.amazon.com/dp/B09FLHR48J?psc=1&ref=ppx_yo2ov_dt_b_product_details

This is the correct plug that you can solder on - https://www.amazon.com/dp/B013JRWCBU?psc=1&ref=ppx_yo2ov_dt_b_product_details

I have been using it for about a month now.