Daniel,
The thing is that the blocks are completely different, and the flows are completely different. Have a look at the parts catalogs and you will see what I mean. Any gasoline motor is essentially a pump, and flow is everything to a pump.
For example, if you put a "stroker crank" in a "grocery-getter" carbureted V8 car, and don't change the rest of the induction (carb/intake), exhaust, and cam/valve systems to match the increased volume, the motor will have worse runnability than if you had left it alone, and little power increase.
Same holds true in 2-stroke OB's. In your example, the block is different, the pistons are different, the crank is different, the reed block is different, the carb is different, (though they share a fuel pump), the magneto is different, the ignition advance curve is different, the recoil is different, the driveshaft housing is different, (though they do share a water pump), the LU is different, etc.
Just not practical or cost effective. Right now you have a 5 year old 2-stroke 30. If you were to replace it with a new 40, it would not be a 40C... it would be a TLDI, which is a completely different animal. Yes, you could hand build a whole 40C from parts, but the price would exceed the cost of a new TLDI. OTOH, if you locate a 5 year old 40, that might be reasonable.
