I don't know if there are any other conversion kits on the market for the conversion, but I thought there used to be. If nothing else, the pics of their kit parts should give you a good start on copying the design. The kit is backordered now. Maybe you could just e-mail them and get a copy of their instructions and then mill your own parts. I wish their kit was cheaper.
Since the original bolt can't be used with the conversion kit, $30 for the trigger housing, stock, and barrel doesn't sound all that bad.
The Tapco Sten kits with the brass bolt are a good deal. You'll only use the trigger housing and internals, bolt handle, end cap, stock, and magazine well in a semiauto sten build. You won't use the brass bolt, but it's a rare and unique item and part of the sten's history.
Building a semiauto sten does require some welding skills, but they're not that tough.