I would use a 337 type passenger car as a start, they are common and cheap $20-30 as is.
A re-gauged marx motor is also very cheap, but McCoy, MTH, Williams and others have made Standard Gauge Traction motors in the past.
G gauge parts are close enough to Standard Size to work for Headlights and Pantographs.
Dry transfer lettering is still made, you just have to do one letter at a time.
I would strip all the old paint, repair any bad solder joints, prime with a spray can primer from HD.
Finish paint with a bright color spray paint from HD.
Remember this a fun hobby not a paid job.
When it becomes work, STOP.