I know this is an older thread but I'll relay what I have found. The Havoc X engine does bolt directly into the Stampede. All of the fuel and firing components are the same part numbers. The engine will however not run on the Stampede ECU for two main reasons.
The bore and stroke of the Havoc X are both greater than the Stampede engine. Stampede 89mmX68 and the Havoc X 92mmX72mm.
Both reluctor rings (crankshaft position) have 34 teeth. The big difference is the Top Dead Center gap on the rings. On the Stampede the TDC gap would have space for 2 to 3 teeth. On the Havoc X the TDC gap would have space for 3 to 4 teeth. The TDC gap is significantly wider on the Havoc X which would cause the ignition and fuel timing to be thrown out of operational boundaries by the Stampede ECU.
As far as a bolt on remedy it looks to me like the Havoc X engine will require a Havoc X ECU. A used one would be the best option if one can be found. A factory ordered one will require a Havoc X VIN. Some new ones can be found but they will have to have the VIN programmed in to be operational (so I'm told).
Questions, comments or ideas?