are the K100 calipers an otherwise simple substitution?
NO!
Many people, including myself have been seduced by the cheap price of the K100 calipers and parts and bought them hoping to fit them to twin shock airheads. It will not work without a serious amount of engineering.
The reasons are, like many things, shrouded in the mists of time. When BMW (and other European manufacturers) decided to go to disc brakes many of them had the problem of previous models mounting spoked wheels, whereas the "new" models were being switched to cast alloy wheels (BMW "snowflake", Ducati - Campagnolo, and Moto-Guzzi - I forget).
To speak specifically to BMW, they made the Snowflake wheel to adopt the same spacing as the previous spoked wheel, even though with its superior strength and rigidity the Snowflake didn't need to do that.
The Brembo F08 caliper was originally designed to mount to wire wheels (Laverda, Ducati, Moto-Morini, Guzzi etc.) and was therefore a no-brainer for them.
Roll forward to the K100.
There was never a wire-wheel version of the K100 so the wheel spacing is much, much narrower - I have a K100 front end on my r65 and side by side the difference is marked.
Consequently the K100 calipers have an entirely different spacing to the earlier F08s. They can be made to fit by either machining the mounts on the forks (not recommended as a good idea) or by making a spacing bracket which restores the old centering. I have seen such brackets for sale on fleaBay, but at a price that makes the whole exercise pointless.
Unless you have machine tools, the costs to adapt the K100 calipers is in my opinion way to high, you could of course go "all in" and fit the 4-spot calipers from the later K100, they have the same bolting as the earlier calipers but would still require an adapting bracket. I actually bought a set of the 4-spot calipers as I thought that they would give ultimate bragging rights, but the K100 calipers work just fine with the 12mm master cylinder and to use the 4-spots I'd have to re-sleeve the master cylinder to at least 15mm.
I did toy with the idea of selling the 4-spots on, until I discovered that they are pretty much identical to the calipers on the GSA so I've retained them as spares for that, or in case any of my K100 projects ever crystallise into a running motorcycle.
Have a look in the misc tech list where I've been posting a running commentary on fitting the K100 front end, I've posted photos highlighting the difference in the calipers.