Which way did it drift off? If high (toward the supply voltage), there's a better chance to get it in range by adding a little extra tuning voltage - the varicap should be fine running a little higher up, as long as there's nothing in the circuitry preventing it. If it's low toward ground, there's less usable correction range since the varicap reverse bias is running out. Still might be doable, within reason. This all assumes a positive tuning voltage that increases frequency in proportion. Ed