With the trade-in program, the 3.3 is a lot cheaper than the 21TM. The last time I bought a 3.3 NIB it was around $130 at a LHS for a pull-start version. Don't know when traxxas jacked the price to the moon on those things... $220... really?
I have a 21TM in one of my revo's as well. It is a...
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...