You can get E3D and Titan clones on Ebay now that are functional. Another way to go is a dual gear BMG extruder with various hotends which is again cheap on Ebay.. My Mendel90 got stripped down for rebuild after a second Melzi board failed. Design changes keep happening so it is waiting on parts again. I replaced the top of the gantry with extrusion for Z space with E3D/Titan combo some years bacby RalphHilton - Mendel90
The official firmware is the one from Nophead that hasn't been updated for years as the Mendel 90 is no longer being developed further. I built mine 5 years ago and it is still running well with the original firmware.by RalphHilton - Mendel90