If the lens is in good condition on your existing fixture I would rebuild it. Your options would be a new T-8 ballast and new T-8 bulbs or you could go with LEDs as suggested. Changing a fixture can run into work. It may be screwed to the joist or it may be held up with toggle bolts. The new replacement may not have the same footprint may not cover up whats behind the old one. (faded paint) Its also sure to be built better than most new fixtures. Equipment from wholesale electric places are usually much better quality than Walmart and places like Lowes and Home Depot.
For what its worth, I was a licensed electrician and probably changed thousands of ballast. Next month we are due to start a kitchen remodel on my own house and I will be reusing my 30 year old 4 ft fixtures. That is if I don't break the lens. I'm not sure if I'm going back LED yet.