What's your budget? Bang for the buck, it'll be hard to beat the OFNA Force .28 that can be found for $100 on eBay as a brand new take-out. Not a super wicked engine, but better than your stock unit, and much more economical than the Picco/Novarossi/LRP/OS stuff.

ETA:

Just looked at your link, and that's not a standard 1/8 buggy. Yours has a small block engine, so a .21 or larger engine will not be a drop-in. To replace with another .18, I'd look at the HPI Nitrostar 3.0. Can be had for under $100, they run very well, and you can get them with slide or rotary carb, and with SG or short threaded crankshaft, depending on your needs.