I am currently converting an old Tower Kaos arf (the purple/white one) for a practice/backup SPA plane. I chose a power setup to stay within the SPA rules. I chose the Emax GT4020 (headsuprc) and intend to use a 11x7 (either 5 or 6 cell). I am mounting my motor on the original firewall with standoffs (cut off the side cheeks). I prefer mounting this way because it gives you a little room to fine tune the cg (that and the battery space).

If/when I get it dialed in, if I like the setup I will clean it up a little (build a custom cowl, wing fairings, etc.).

Right now I am in the process of cutting a battery hatch. From what I can tell, its going to be a big hatch, but I would rather have it too big than too little.

p.s.- I agree with everything doxilia says.

p.p.s-Thanks for the Vagabond link. I have one, was going to give it away. Now I may have a new project. Beautiful plane.