Just fitted a spare m20 engine into a friends plunger model because his mains were gone but it's really gutless compared to the old motor' Did all the usual diagnosis. The only thing that is different was fitting a 2 inch manifold spacer for the monobloc to clear the generator which the old motor didn't have.Does anyone think this could upset things

Hi Douglas...That shouldn't be a problem..a spacer of this type was fitted as standard to M21s and the lengthening of the inlet port actually improved the lower end as it was nearer in length to the theoretical optimum.
Assuming timing settings, carb settings etc. are correct the most likely cause is a difference in compression resulting from tired valves/seats...that is by far the most common cause for weak performance.
If you get it on compression and stand on the kickstart it should just about hold your weight...assuming you are of average weight (I am about 14 stone..196lbs.)....Ian

email (option): julie@wright52.plus.com

Yes as a matter of fact the spacer was off his M21. We'll have to go deeper i think.