The Savage Octane engine is 15cc vs 7.7cc for the CEN. The 15cc is going to have a wider crankcase. Figure on using the Savage engine mounts which thankfully attach to the chassis on a flat horizontal plate. Maybe you'll get lucky and just be able to drill the CEN engine mounting plate but you'll likely have to make a new one. The thing I'm a little worried about is the exhaust. The CEN chassis has the engine on the left side of the chassis in the Savage Octane the engine is basically centered in the chassis. Since the exhaust exits the right side of the engine the exhaust may need to be extended to reach far enough to the right side of the CEN chassis. This requires some cutting and welding. The rest of the fabrication is fairly simple, drill holes, make brackets to hold the CDI box and fuel tank in it's new location.

The Losi XXL2 engine should be a bit easier. You'll still likely need to make a new engine mounting plate but the fuel tank can stay where it is. The exhaust will be easier too, just swap the XXL2 header for a buggy/truggy header, just be sure to use the XXL2 tuned pipe not one meant for a nitro.