Using a 2.6" Hawk model, can an e-bay be added?

Ok, I must admit I really am having some beginner jitters here.
I have bought two of the Madcow 2.6" fiberglass Hawk kits.
They are really nice, but I would like to extend the rocket length and add an e-bay to them both.

Problem is I really dont have a good way to determine the effects on stability.

I also have a little trepidation about the rearward location on the engine mount.
How far back can I place it, and will that affect the CP?

Thanks from a new guy.

You should be ok to do what you suggest. Lengthening the body with an AV Bay will only bring your CG forward, which can be a good thing.
If you want to find CG/CP and be able to figure out your planned mods stability, I would highly recommend downloading Openrocket so that you can Sim everything first.
Where you mount your MMT has no effect on CP, but it will on your CG. Most kits of this style tend to have the MMT located deep into the rear of the body tube to help keep the CG forward to overcome the effects of the forward CP.

Thanks for the reply.

I will have a look at openrocket.