I would think that you should start looking for another box van for parts. Do you realize how little demand there would be for such parts? Even from Corgi. They have to have a large assortment of cars and trucks to sell them in the first place. Then, when they discontinue a model for a new one, where would they store the thousands of teeny tiny parts?
I only see the guy's honest suggestion, his best guess and then a dose of reality.Though rather blunt sounding, imo it can be read without emotion, and no real malace; or with it, maybe some patronizing tone at best, but still just facts some folk may not consider. It's really up to reader's state of mind how it reads. It did start with the best avenue likely to get you parts cheap and fast.
Take a plastic model sprue and stretch using heat to get the taper, then cut: you have a bugle. Drill another chunk of sprue to accept the bugle; round the other end. You have a diaphragm case and bugle now.
You can make car antennas and windshield wipers/arms with some super glue.
Or sand a dowel into a taper. (+ Exhaust..or tube for exhausts, wire for pipe, thread strands for motor/dash/light wires. Glue on the thread, dry/semi-dry to shape, cut, apply for tough ones)
5 min JBWeld can be wet finger molded/sculpted with tools, etcafter about 2 min., until you see stress cracks starting.(similar to Clay drying out)
I replaced a nose on this swan below with model putty. (I think I prefer the JBW, at least at first. Putty takes months to harden well imo. It doesn't sculpt as smooth either.
Mirrors, wire, super glue, and a snip of aluminum foil box teeth; to shape
The smokestack on this kids Vulcan is about 1/8" tall, JBW rolled in two fingers and snipped with scissors, patted flat on top with my finger. A horn wouldn't be much harder imo; I think it looked like a horn for a while while being rolled.