Crossing the Atlantic in a 21ft boat is not so strange at all! Just look at the guys crossing the Atlantic from Europ to Brasil in 6,5m boats! The Unofficial Mini Transat Open 650 Site
I crossed the atlantic on a 50ft boat just a year ago, my opinion:
Fuel? Not a problem, you sail!
Electricity? A hydrogenerator (like the one from watt&sea) and/or small solar panels are enough for radio, light, iridium and ais for the night.
Food? you will fit it in. You cook with gas. 2 bottles will be more than enough.
Crew? Would not do it singlehanded, but with 2 you should be fine!
If you go e.g. Las Palmas to Martinique you can have an easy sail with the trades, just do not forget your spi and a good storm sail, just in case!