The Tornado stock is a side exhaust rotary carb.  The side exhaust is pretty limiting when looking for upgrade engines.  The SH 18 is a decent basic engine.  I have one, and it has been reliable and easy to tune.  But it only delivers 1.0HP and a max of 29,000 rpm according to the SH specs.  The OS 18CV-RX is one of the few upgrade side exhausts and delivers 1.35HP and a max 32,000 rpm.  If you go to a rear exhaust, you have more options and can find more power, but you will have to replace your header at a minimum and probably your pipe.  In a rear exhaust, you can find numerous upgrades with more than the basic 3 ports and more power.  The SH 18 rear exhaust engines deliver 1.35HP and a max 32,000 rpm.  The OS 18TZ rear exhausts deliver 1.8HP and a max 32,000 rpm.

To go to more displacement, you are looking probably at a 21 engine or maybe a Traxxas 3.3 (a "20").  Any of these larger options are again likely to be rear exhaust requiring an exhaust change.  And as "big block" engines, they are also almost certain to require modifications to the mounting including the slots in the chassis and/or the motor mounts.

Bottom line - SH18 is a good basic engine.  If you do that, use the engine exchange suggested by Derek which really saves some $.  The OS CV-RX is a nice upgrade in a side exhaust.  If you do that, I would suggest your LHS.  They can usually match mail order or internet prices, especially after shipping charges.  And they are more enthusiastic about supporting you if they have seen a few of your $.  Next step up would be one of several different rear exhaust engines.  Next would be a big block upgrade.

Good luck.

Bryan