just another point of view-but these became the 'weapon of choice' for sidecar cross in the late 80's and early 90's.
Consider that the engines of the day were 4stroke twins with about 50-80HP but they weighed 40kgs or more.
Bring on a 500 2stroker that has similar HP but weighs a shite load less and you have a winner. Then 'strech' that motor in all directions-longer stroke as well as bigger bore was not uncommon-and it became a monster.
There were plenty of Hondas but also a number of KX's and KTMs. Not too many YZs or RMs but a few Maicos, particularly in Europe. The Maico motor became the basis for the Zable which is now a brand of its own and winning the GPs. It has been known to be 700cc or even bigger and over 100HP isn't out of the question.
Luckily that it to tow two guys or the pilot would end up with very long arms.