if they aren't glass matt
I'd bet you can get one in the USA cheaper
many for alarms are in the ball park of the right size and you can pick them up local for less than 25 bucks
glass matt........ they are a bit pricey
Craig Vechorik at Bench Mark Works sells a similar battery.
They're not as big as the original /2 batteries, but fit well in the tray.
If you need a "square" battery for a /3 or earlier, they are available, not any more expensive, but not well shaped for the battery tray. They tend to be pretty thin in one dimension. (See Model SLA-10-6.)
I've cut up an old battery to hold one of these, but it has to stick out one side a little bit. Not really noticable on my R51/3, but odd when holding it in your hand.
I have an SLA from Vech and a regular Yuasa, both spares, not installed. The SLA battery is 1/4 inch less in depth and width, and 1/2 inch less in height, so it will need some blocking in the tray. The SLA will also require cutting off the eye connectors on the bike's wires, and putting on female spade connectors.
you might be surprised how cheap you can find one, appropriately sized for a vintage bmw, just right down the road from your front door
many sized for non-estart bikes are in the 9-15 buck range
but glass matt.......... they are awesome but quite pricey
on a side note, I'm not the least bit impressed with the very under-engineered battery isolation on these bikes
hard to over-insulate from vibration and very inexpensive to do so without it being noticeable
for sure one can procure modern high density foams which are far superior to the thin molded pieces of semi-live rubber
http://www.batteriesplus.com/p-32644-we ... minal.aspx
They work great, are inexpensive, maintenance-free, and readily available.
I like the "s" connectors, which allow you easily to unplug and charge, or remove, the battery.
It's a little shorter than stock, so I add a small pad under it.
To match stock, it should approximate 6.5" (16.5 cm) high, 3-1/8" (7.9 cm) deep, and 4-5/8" (11.7 cm) wide.