1. Left hand bolt mod
2. Side plates to simulate stamped receiver
3. Big ejection port
4. Ejection port cover (maybe from AR-15)
5. Left hand safety in pistol grip assembly (maybe just use parts from H&K lower)
Depending on how crazy you want to get with this, there are bead rolling tools for sheet metal available. They're not cheap, but some bodyshops have them for stiffening the aluminum interior panels used in race cars. If you're on good terms with the shop owner, they may let you play around with the bead roller. That'd be the best way to get the ribs in the side panels.
Links for pics that I found:
Link to page with the MP44 pictures: (These are GREAT closeups and details of internals)