I've tried looking at the replacement reservoirs using a Google search. All of them show tabs with holes that I assume allow mounting by bolts or screws. Some I have encountered on other vehicles slide into a wedge shaped slot and just lift out. Since yours is unusable, I'd get it out by any means (after photos and measuring it) to figure out it's mount. Then take the remains and go shopping.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI