any hard ware store will have good oil based enamel for metal. in australia id say look at the rust-guard or kill rust range they are white-knight(PPG coatings) and wattyl brands respectively. rustoleum is also good they all sell small tins and you can buy base tins to be tinted to your choice of colour. i think mailbox red or fire engine red would be close.

if you want a neat job get some fine line masking tape all good paint shops and autoparts have it, some hard ware stores do too and a craft knife(exacto knife) you can also use the normal 3m painters blue tape and the knife to cut outlines but its more fiddly.
the secret if using a brush is load the brush properly and use a single stroke for each part of the letter, paint in the shade on a cool day and and if using oil based paint warm the paint(stand the tin in a dish of hot water) or add a little penetrol.

paint pens must be stored flat and shake real well before and during use.

Tom,
Sponge roller, good paint with hardener will work well. This has been around for many years in restoration world.
Have a good day......Bill

I'll attach a few pictures of the progress once I get a few more coats of paint on the lettering. I wound up using a foam roller and it went on suprisingly easy once I played with it for awhile. The roller pan I used left little spots on the roller and wouldn't leave a consistant thickness of paint on the lettering. when I rolled it on a flat surface, it distributed the paint on the roller and applied a nice even coat probably a few mills thick. It will take 4-5 coates with 4 hours between coats to get it looking good.

BTW, would anyone be interested in a picture of most of the Caterpillar service awards issued over the years? I'll attach a picture of that as well if anyone is interested and don't consider to far off topic.

Tom

Thanks for the pic of the beautiful Cat Thirty.
I will attempt to post a few pics of my Cat Thirty with it's cold weather package.

Tom   

OOPS, didn't mean to post twice. 
Sorry!
Tom