FUEL PRESSURE REGULATOR

What about the four plug in sensors on an airbox? What are you doing with them? It isn't just about fuel pressure.

Without the airbox, Fuel Pressure will not be a problem.
It will always be 3 Bar above atmospheric; Which will be correct.

The REAL PROBLEM is that without the Air Box, you will not have a Throttle-body on the 912is Engine.
And without the Throttle-body, there will be No way to Throttle down the engine.
It will ALWAYS be at Wide Open Throttle (WOT)!

DO NOT attempt to start the Engine without an Airbox!

Also, The Throttle body is Spring-loaded to WOT.
The throttle must be intentionally closed or the engine WILL go to full power immediately on startup!

Be sure you understand the implications of what you are doing.
What you are considering is extremely dangerous!
The threat of Fatal consequences is not an exaggeration in this situation.