Rabid...I am not sure how much of an engine upgrade you want to put on it as the model is not all that sturdy. the mfgr gives it engine ratings for a reason. What you may wish to do instead is design a new rocket with an interchangable motor mount so you could run a single 29mm, single 24mm, 3 and 4 cluster of 18mm, 2 or 3 cluster of 24mm motors, etc... You could make it for a drop in style motor mount and have fun while designing it.
It would give you a large variety of things to do and should keep you busy for quite a while.
Carl