As told, I wanted to replace the 3-speed hub with a 6-speed shifter. Another auction won at ebay brought me a 6-speed lever and a matching derailleur, "SPS"-Type from the 1990s. The derailleur does not fit to the frame without an additional part: I need a mech hanger. If you image-google for such part, about 200 different types appear… I went to my local bicycle repair man, who gave me one from his shelf. And - surprise - it was just the thing. I have linked the derailleur to the hanger by using some nuts, bolts and washers. A real DIY job, but very reliable.

The mech hanger and the safety-nut-and-bolt junction between the hanger and the derailleur (at the top of the picture)

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Mounting nut, mech hanger, derailleur in place

Unfortunately I had to re-respace the rear wheel a little bit to avoid the gear cluster rub to the mech hanger mounting nut.

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That looks good.

The frame is also missing a cable stop for the shift cable for the derailleur. I diy one from a nylon part and mounted it to a remaining chainguard bracket at the chainstay. That looks good also:

The diy cable stop mounted to a metal bracket (which is the former chain guard holder)

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The next problem on a never-ending list: When running at gear 5 or 6 (the smallest cogs), the chain would run exactly over the top of the bracket and will seize up there when pedaling. With massive pliers I snapped off the chain guard bracket, filed the metal down and covered it with some varnish.

I found a shop with NOS cycle parts (link shows the cable stop) and have ordered missing things, also the cable stop.

A further issue showed up while testing around with the chain. The Kettler chainwheel does not like the chain, which is too narrow! The chain is a CONNEX 808 (1/2" x 3/32"), 2,4mm narrow type. The Kettler Chainwheel has a 2,8mm width (for 1/2" x 1/8" chain). Well... I have to bid on ebay again!

For a bargain price of 24€, I got a Shimano FC-M300 crank arm set, including a Biopace 48-t steel chainwheel mounted, both in good, used condition.

While waiting for delivery, I will mount the brakes.