The Marin County guys, who get most of the credit for starting the modern-day mountain biking industry, rode old Schwinn Excelsiors (mostly) which they modified to stand up to the rigors of off-roading. These bikes were usually referred to as "Klunkers".
In 1997, I decided to build my own version of Gary Fisher's Klunker, which he had outfitted with a triple crank from a touring bike, drum brakes, a multi-speed freewheel and derailleurs. Of course, the older parts that he had used were pretty much impossible to find in those pre-eBay days, so I bought new when I had to.
The triple crank was no problem. We had an old touring triple in the junk box at the shop. I ordered new Sachs drum brake hubs, found an old bullmoose-style bar and stem combo, etc. I hung all of the parts on a 1940s Western Flyer frame that I had picked up at some point.
When it was done, it was a pretty fair facsimile of Fisher's bike, including the 40-pound weight. But, it rode pretty nicely on the trails. I actually rode it to the top of Bergen Peak on a club ride, one weekend, because I had loaned my mountain bike to a to a customer at the shop.
It was easy to see why the Marin guys got so excited about their "new sport", back in the day, after riding the Klunker up and down the mountain. Especially after years of riding single speed bikes and ten-speeds off-road.
It was not too bad, really, even compared to a modern mountain bike. And, that is one reason I went back to riding a rigid bike. It makes me feel a little more in touch with the roots of the sport.