Hello & Welcome to the Forum!
Bashing a locomotive can be fun or drive you crazy.
Easiest fix: find a junk tender, introduce it to Mr. Razor Saw, affix the piece you cut off to the cab of your loco. Voila!
Craftsman fix: get out your napkin & pen, design the fuel box, remembering the iron horse needs h2o also.
Get out the sheets of styrene, lay out the pattern from your napkin, cut the pieces. Using the glue of your choice, put the pieces together, sand, attach to cab, paint, celebrate....
There's a photo of one of my large scale 0-4-0t bashes in the gallery.  I'm a fan of 0-4-0 & 0-4-0t locomotives, have 10-12  large scale by various manufacturers & bashes.
I hope this is of some help to you.
Happy Rails!
Dave