Exactly why I don't get why you are constantly hang these bikes out as handgrenades. It would be - business-wise and also in respect of the owners of such bikes - much better to point to the problems but offer help through OCT instead of banging permanently on the issues, Piet.
Personally, I don't believe in bullshitting my customers, or anybody else for that matter, ie, I provide them facts when they inquire about buying these bikes. I tell them straight out what they're letting themselves in to, up to them alone to decide if it's worth it. What do I gain by encouraging somebody to buy such a bike and invest "an additional small amount" to "put things right" and it turns out to be "a rather considerable amount", far more than the bikes' market value? After a year, it's another "small investment" and another, and another. You know full well how disastrous these bikes are, even when the major issues have been attended to. There's the paint quality of the wheels with the accompanying corrosion, quality of rubber bits that deteriorate with alarming regularity, lousy lighting equipement, just to name a few. You will again argue all this can be attended to... of course, but you really shouldn't need to! Not everybody is prepared to sink as much cash into their bike/s as you and I. Other bikes of the same era and similar pricing played in a vastly superior league. There's a reason these bikes did not sell in great numbers, the motorcycling public isn't entirely dumb.
Quite OK if a 40-year old bike has loads of issues imho, this can be expected. But bikes that were troublesome while standing on the showroom floor? No thanks. Most other marques managed to get their products through the warranty period without too many hassles, Of course, there were a couple of notable exceptions, but, on a whole, IMoLa S.p.A. couldn't hold a candle to their competitors. The Zane factory should never have thrown such an under-developed and shoddily-made product on the market.
Please note that I was not involved in sales of these bikes when new. My experience with them at that time was limited to an extended evaluation ride on the first 650 Sport prototype that came to Germany, seeing the first UK 650 crash in front of me in Wales due to a cracked handlebar clamp and a ride around Zane and the Dolomite foothills on a pre-production 750 Sport. The 650 didn't last a week in the hands of the Moto Witt staff (all experienced Laverda riders), the partner machine that Roger rode in Italy didn't make it out of Zane due to overheating. While not having ridden as many miIes as you Lothar, I have certainly ridden many more Zane products, nearly every single model. I became deeply involved long after issues arose and have helped Roger develop cures and work-arounds to keep these things running. We both see it more as a technical challenge than a source of income.

Sure, it throws off a few bob, but the effort often simply isn't justified.
I have to admit, these engines are not to my taste, which does not mean I don't give them my full attention when working on them. From a 650/750 4-stroke twin I'd expect stump-pulling low-down torque, not an engine behaving more like a 250 2-stroke twin. It's impossible to trundle through town below 3500 rpm due to lack of flywheel and miserable fuel management. Reliable sources reported Zane had evaluated a Yamaha TRX 850, why Zane didn't take more notice of it is beyond me. Yes, the TRX was quickly demoted by the up-coming litre bikes such as the Honda VTR and Suzuki TL models, the Lav didn't even stand a chance.
Sure, they're fun when fully sorted, but it is neither easy nor cheap getting there. For every bike still running, a dozen have been broken and flogged off in bits because an owner has surrendered. If somebody thinks he/she's
up to the task willing to pay, OK by me.
piet