Here'll be something simple. Easy simple way to make a head HT compatible?
You could use a dremel to cut hollow out the bottom of the neck to fit a HT neck joint (if you head has a neck) then super glue it together. If your head does not have a neck then you can try hollowing out the bottom of the head with a dremel and this attachment.
https://www.lowes.com/pd_94695-353-107_0__?productId=1114213&Ntt=dremel&pl=1¤tURL=%2Fpl__0__s%3FNtt%3Ddremel%26page%3D2&facetInfo=
Start by going straight in. then move around in a circle on the inside until the hole looks big enough to fit. Stop frequently to test fit. If you make the hole to big then it will be too loose. I would practice on a less important head if possible.