Hi Hugo,
I forgot the B26 folder has two aircraft in the one folder, the B26g and B26c you need to change the actual model files for the plane that is missing in this folder.
If it was the B26g thats missing after patches, then look inside your backed up folder on your desktop (or where ever you saved it) and you are looking for the files that have B26g in there name and the file type is .M3D
b26g.msd
b26g_cockpit0.M3d
b26g_cockpit1.M3d through to cockpit model number 6.M3D
etc
the other file that you may have to include is the B26g.air file, as I think the updated one is possibly set to 2 in the aircraft type which would render the plane invisible in the aircraft list.
select all these files and copy them into your b26 folder in your game and allow to overwrite.
if its the B26c thats missing then same again as above but grab the B26c files of the same name.
hope this helps,
regards Rob.
PS if this doesnt work PM your email address and I'll get you to send me your updated B26 folder and I'll see if I can work it out for you, then send them back to you.