In my opinion i would say make a whole new engine. However making it simple and easy to use like Mccodes v2 and easy to make new modification
If this is the case you shouldn't launch the name as Mccodes V3 but more of a new generation, Right [i.e Mc Codes New Gen 2012 ? xD] ?
In any case, would go for the toally new built engine then working back on mccodes.
(My english sucks as always, still trying to improving =/)