Higher compression with the 2 shims and the HC glow plug, I use small props for speed and 1 shim with a HC glow head.
NV is a fine thread for honing down the settings to peak. Choked down venturi after a previous boring out gave better fuel draw
with the right prop and good outdoor temps the engine will sing along happy and steady. Looks like a Cox 4.5x4.

Do you recall what brand fuel line? I had some back in the day cant recall now, the ID is good size for .049's.

Lower castor % providing the temps outside are okay will actually give more rpm with a good fitted piston / sleeve.
Too thick oil slows down the rpm the car guys like to get away with all synthetic for better top end for that reason and to avoid build up.
4cy engines cant use but a small amount of caster the nature of the engine design causes build up in the case to the point where it will gum
up the bearings and not turn over without preheating the case.