I agree 100%. Have run into the same problem frequently - kit manufacturers place the firewall away back to allow for the longer four stroke engine. The two-stroke majority are thus forced to find some way to accommodate a bad engine fit.

What the two-stroke owner frequently does is to use a long mount, which is naturally more flexible, and that may go into mechanical resonance with a high RPM two stroke. Flexibility in an engine mount is a two-edge sword. A huge amount of flex is good - as in soft mounts. A rigid mount transmits virtually all the engine vibration to the airframe, but this normally does not cause a lot or trouble.

A mount with just the wrong amount of flex is very bad - it can go into resonance at peak RPM, transmitting several times as much force to the airframe as a rigid mount would, and this is frequently not detected during ground running checks, because the engine unloads to 1,000 or more RPM in the air. An engine and mount combination that goes into resonance in the air shakes everything brutally. I managed to diagnose this problem by installing a prop with one inch smaller diameter than was being used for flight, whereupon the engine vibrated extremely violently at full RPM on the ground. The engine was reaching roughly the same RPM on the ground as it would have attained with the normal size prop in the air. When using plastic mounts, I think that it is best to place the engine well back on the mount. If the engine will be too far to the rear, move the firewall forward, or shorten the nose ahead of the firewall.

Four stroke engines are fine if you just love their sound; otherwise they are best avoided.