Hey maki, good to see you here as well besides HB!
If you're based in the Netherlands I maybe have a tip for replacing that 3l water bottle which you use a a water supply with something with a nice large filling opening and the possibility to install a debris filter:
https://rotterdamplastics.nl/product/watertank-5-liter/" onclick="window.open(this.href);return false;
This is how I enlarged the capacity on my Linea Mini using a t-section on the tank line... see the tank to the left of my machine on the photo below.
Also you might want to contact these guys:
http://truebarista.nl/" onclick="window.open(this.href);return false;
Wouter (they're a couple) completely restored a late 90's one group linea which they use at venues. So he'll know the inside outs of such a machine and maybe able to help you upgrading it.