Is this the piece that is pretty much a plug 'n' play replacement for the rear brake system on a K1100LT?
https://www.ebay.com/itm/Motorcycle-Rear-Hydraulic-Foot-Brake-Master-Cylinder-Reservoir-Dirt-Bike-Pit-ATV/312810734373

I used the one with the elongated mounting holes. See here for how to do it there are a couple of ways. Some people have used their OEM hose, some have used an adaptor I fitted a new hose. http://www.motobrick.com/index.php/topic,9364.msg115727.html#msg115727
Regards Martin.

That looks like the correct type of master cylinder.  The thing you want to check is that the distance between the mounting holes is the same as the OEM unit.  I seem to recall that the distance is 54mm and the master cylinders with elongated holes as Martin has mentioned have the correct spacing.  Best to measure yours.

The good news is that if your brakes are stock, the line will have a banjo fitting that will work without modifications or adaptors.  Early model K bikes had the line screw into the master cylinder, and the Chicom threads didn't match those on the end of the brake line.

You want to replace the line from the reservoir with one that is rated for brake fluid.  There is a report that someone has had this line fail because the fluid ate it.

Outside of the reservoir hose, the only thing you need to do is modify the push rod.  On the three I've done, I ran the nut down until it pressed against the rubber boot.  I cut the pushrod at that point, and then countersunk the end of the rod with a 3/8" drill bit.  The combination of the countersink and the nut forms a pocket to contain the ball end of the pedal's pushrod.