I have been pounding the internet for few days now on this subject, but I guess like you have not found much. However there was a guy on a video that just run the biogas hose into the carburetor of his gas engine and it run fine, with a valve as a regulator. Now I know for a fact that a diesel engine will start on starting fluid and that is a gas vapor by the time it will get to the engine. Statring fluid will get the air cleaner wet but it is a vapor going into the engine.

Have you tried to run your engine on diesel and then put the biogas hose in the air cleaner and see if it reves up. If it does then try shutting off the diesel fuel and see if it run on the biogas alone. I was reading a web page that showed an engine running on duel fuel (start on diesel then switch to biogas) and they said that there was carbon build up on the pistons and exhaust valve but it was negligible for a 2000 hours test run. I did not see anything in the study if it was special engine or not. I am assuming it was a normal diesel engine. They recommended further studies.

You would control the RPM's by the flow of biogas into the intake via a valve or regulator. Remember to much starting fluid in a diesel engine could cause sever knocking and possibly blow a hole in a piston, or two. It could also twist the crank in 2 pieces. So for now just wave the biogas hose by the intake and see if it reves up first. let me know how it went.