Firko,
To give Yamaha full credit the MX was meant as a 'Play Racer'. "If you wanted a serious Yamaha MX'er buy a YZ" was Yamaha's policy. Thats why the MX was over weight and over engineered, came with oil injection, a 30mm carb and 'soft' power.
The MX was a Play Racer par excellence - first and foremost, cheap and reliable. The fact that it could easily be made go hard was the hallmark of Yamaha's two stroke knowledge - they didn't know how to make a bad two stroke.
I reckon if you put a Maico front end on, a lightweight rear brake/hub and some decent shocks, you won't pick the difference in proformance between the MX and the Europeans handling/chassis not withstanding.
If you used chromemolly steel for the chassis and paid attention to weight savings you would outperfrom the European comtempories. Hang on. Doesn't that describe a YZ
.
If Yamaha made a mistake it was in the Marketing dept not the Engineering dept. Marketing correctly identified the 'customers requirments' but didn't correctly identify the 'customers needs'. They were a bunch of wankers and they 'needed' to look like their heros even if they couldn't ride well enough to do justice to their hero's bike.
Yamaha realised their mistake and went on to do the opposite (as did all the Japanese) - sold bikes that looked the same and were called the same but were like chalk and cheese.