I have bought myself a F20T12 fixture and it has a Magnetek RS ballast in it but I want to convert it possibly to preheat, what are the suggestions of what ballast I should use?

Well i have some good news preheat ballast is Getting expensive  and harder to find.   First measure the voltage between the red and blue and see if u get at least 200 volt or better open circuit.  If this correct then it be estentally a Single lamp F40 ballast with extra
 Cathode heating.  What u do disconnect yellows and one of red and
 Blue wire.   And put starters across each pins on each end of the two lamps.  Just wire rest in series.   Then u got your self a series
2 lamp preheat out of a rapid start ballast.