My idea is to buy only the MLA30+ mag-loop circuit (that I wasn't able to find standalone)

Amazon has 'em, for around 6 bucks US. Shipping takes about 3 weeks :

https://www.amazon.com/dp/B09NB4ZZCQ?psc=1&ref=ppx_yo2ov_dt_b_product_details

Oh, it's a kit, hope you know how to solder...