Depends, there are more than 1 I think.
Size wise, yes. As for the other amenities, don't know. Such as, slide or rotary carb, side/rear exhaust, threaded/pilot crankshaft...
Depending on the "maxx" and on the engine, if it has the right stuff listed above, then it will fit without a bb kit or anything. If it's rear exhaust, he will need to find an alternate header if he's replacing a 2.5/3.3 as traxxas doesn't use a standard exhaust output port.
EDIT:
Just checked ebay and all the SH 18's appear to be side exhaust with a threaded shaft. But they did all appear to be a slide carb. I may have been remembering an old XTM 18 Pro that was rear exhaust...