It is best to have a mounting system which absolutely locks front of engine and forward prop shaft into a rigid setup -
There must be NO chance for the engine to have any "wind up " in the drive belt -thru wobble or tension change on impulse.
4000 engine rpm should not be considered "low"
Once you use flywhel and build up to a heavy structure - you get into a very heavy power to weight setup.
We have done these setups -using cog belts (on smaller engines ) and they do work.
the system tho must be rigid tho -
yours looks to have possible "windup ".