Dear Taeuss,
Good to hear from you. Being a creative modeller you can of course use a replacement head of any other brand like the ones with more dynamic facial expressions from e.g Alpine. Rado figures matches the sizes of generally available resin figures so that shouldn't be a big deal. As for the Sten it's not about the magazines but about the pouch the insurgents used everything what they could get and that was the intention for this figure, but of course, as stated in the description text, you can use the alternative MP40 that comes with this figure.
Best,
Rado