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For the record, I have a 1982 K1 with a Yuasa YTX12-BS 10.5Ah battery which is supposed to have CCA 180. It fits OK and turns the starter but does not 'spin' it and so is NOT a reliable method of starting (it starts pretty much first time using the kickstart). I looked for an alternative but most seem just a bit too wide.

Just a bit of an update. The battery I originally tried in it was a 15913, which fits several BMW's, and fits the carrier perfectly. This is 18Ah but the standard one is only 100A CCA, which isn't enough to start the engine. It will turn over reasonably well, but I think what happens is the terminal voltage drops to about 8.5v, which maybe isn't enough for a good spark. Anyway, I've now bought a 15913 high power Exide AGM12-18, which is the same physical size, 18Ah, but 250A CCA! Ive just tried several cold and hot starts and it works fine.
Thanks to all for info.

I am pleased to hear that you have found a battery which fits and does the job.
My experience of an Exide battery was poor in that it failed after a year or so. I hope that you have better luck!