There seems to be a good selection of modifications done by owners in the US and Canada, here is one example.

https://www.gatewaytoairguns.org/GTA...topic=122164.0

https://www.gatewaytoairguns.org/GTA...topic=122165.0


From the UK, one with an extender to aid cocking and one with a stock and silencer.

https://www./community/index.php?thr...v-hw40.271022/

https://www./community/index.php?threads/hw40.262665/