Give me the dimensions of the machine ie: Profile lengths for X and Y axis and I'll see what I have kicking around that could possibly send your way. I've got several second-hand but in good condition linear rails and also some new 15mm Hi-win bearings floating around. I've also got a few ballscrews floating about.

How is the gantry made up it's hard to see from that picture and I've seen them build these things several ways, plus you said you'd upgraded it. Just trying to determine if it's strong enough and best layout for rails. etc.

Regards the Z axis and C section profile and it sticking out a little far then I wouldn't worry too much because if you change to linear rails then the gantry end plates will probably need changing so you can design them so the gantry is offset back a little to reclaim the lost travel.

Give us better pics of the machine from different angles so can see better how to beef it up.