Why replace the whole hood just because of a ballast - go to a lighting shop and buy another ballast.
Ballasts are spec'd according to wattage only - so if you are running 20watt bulbs then get a 20W ballats and swop it out.
Also, dont overrate your ballast. By that I mean dont buy a 50W ballast to run a 20W bulb. The ballast will pop the bulb eventually because it causes too much power on startup.