Originally Posted by floeckx
The only thing I can spot are two small holes in the lying rear fork.Frederik
Assume you're refering to the left chainstay.
IIRC those are the standard Hayes rear mount (same as that on my '02 Trek 8000) on which rear Hayes mag calipers would bolt directly.
There used to be adaptors which converts the Hayes Rear Mount to International Standard like this:
A2Z.
There are universal bolt-on disc brake mounts which may be a better alternative, example:
retrofit.