There was a company a long time ago that made some semi stens "catco" I think. I don't know how well they ran. I thought about doing one when the MK II kits were dirt cheap. I was talked out of it due to what a PITA it would be to drill out the hardened bolt for a firing pin not to mention trying to figure out how to tinker with the fire control to make it semi only. With enough money I'm sure anything is possible, but would it be worth it? Sounds like a suggestion for an episode of "sons of guns"
I have seen some semi Sten kits out there but never a completed gun. If you do a google search there are a ton of kits out there and complete guns. I bet the quality of the different kits vary greatly.
Personally if I was looking for one I would look for someone that has one that works and then try to buy that gun or get one manufactured by them. Its better than buying something and building it and then find it doesn't work. I have shot other guns that were made from kits and such and some work and some don't.