Another 3.3 most likely. Not many drop in alternatives left. Might be able to find a dynamite 19 or the OS 21TM, but if you enjoyed the 3.3, use the engine replacement program with traxxas so your not spending $200+ on a cheap engine.
There's also this one, don't know anything about it though:
https://www.ebay.com/sch/i.html?_from=R40&_nkw=traxxas engine&_sacat=0&LH_TitleDesc=0&rt=nc&_udlo=100&_udhi=500
There was also a tecnopower .26 drop in that I think came and went in the past 10 years.
Kind of stinks really. No more RB's, picco, novarossi drop ins...