Welcome back! A lot has changed in model trains in the past 20 years, but you probably already know that. You can still find tubular O in stores, but not sure about O27. If you want to go that route, you're best off going to train shows, flea markets and searching on ebay for the used stuff. K-Line made great-looking Super-Snap and Shadow Rail that is compatible with Lionel's tubular O, but they were snuffed out of existence by Lionel and their track is now hard to find second-hand (however a company called O-Line Reproductions makes a modern matching version of the switches). All the major manufacturers are now going with more realistic rails and ties and custom connectors (pins), so they're frequently not compatible with each other. As far as assembly goes, mostly still the push-and-pull method (Lionel's Super O had both pins in the track and male & female locking connectors on the roadbed).
So if you want to continue to build with tubular, hunting for decent-looking second-hand track is the way to go. If you want to modernize, Lionel's FastTrack (with snap-together plastic roadbed) and companies like Gargraves, Atlas O and others with more realistic rails and ties are your options.