There's no point. SpaceX starship returns on its own, so taking a mini-VentureStar (which is what X-33 was), making no sense.
X-33 was powered, or would have been powered by linear aerospike motors derived from the J-2 motors of the Apollo program. Once Venture Star and X-33 were nixed, the engines were again tested, this time back with regular nozzles (edit: by this i mean tube wall, ablatively cooled) into what would have been the J2-X motor for use in the Ares V and Ares I rockets for the Constellation program.
But they never could provide enough thrust. NASA vacillated on going to some kind of SSME based upper stage instead, though that was even more problematic as there had never been a method to start the SSME's without a great deal of bulky ground support equipment. I'm not sure if they had even made their mind up before the entire Constellation program ended after the Obama administration scrapped everything but Orion.
some of the variants were weird. The standard Ares V was supposed to have, if memory serves, a lower stage with two shuttle derived solid rocket boosters and five SSME's. The upper stage would have had a single J2-X. But there was also a variant nicknamed Longfellow, much taller. Using 5 RS-68's from the Delta IV rocket on the first stage,, long upper stage with a J2X, 8.4m diameter. The SRBs were five segments. There was also the side-mount variants, harkening back to the old Shuttle-C concept (ones I favored and still do). Anyway, this is an aircraft discussion and i am corrupting it with rockets.
The Venture Star was always going to be a commercial program if it happened. Lockmart had a lot of troubles with the odd shaped tanks the vehicle required and it just didn't work out. X-33 probably just attempted to do too many things at once. If Delta Clipper X had been followed on instead we might be where we are now but a lot earlier without all these relentless false starts over the last 20 years.
