You'll find that you miss the mid-range grunt of the old XL with modern saws, but will enjoy the "high rev, cuts smooth" thing when you get the hang of modern units. If you want the 20" bar thing because you cut a lot of bigger wood, the 50-53 cc class will be a little underpowered; if you like the 20" bar to save your back from bending so low and cut mostly smaller wood, 50cc will be fine. The Stihl 361/Husky359 (and other makes in this class) are a wondeful compromise for most...they're lighter and handier than the 70cc class saws, and have more snort than the 50cc size. If you have an EFCO dealer in your town check out the 962...a real snappy saw; John Deere also markets this saw in green.
Keep in mind that the old XL/super XL has been around since about 1963 without any major design improvements. The last ones were cleaned out of warehouses about 30 years after that, and they are still to be found everywhere, used. If you really really liked that old, slow, "buzzy" Homelite I'm sure a cruise of pawn shops, junk stores, garage sales, and even Ebay will net a few...