I have just cut the firewall in my Katana for a Roto 35 from RCS showcase. If I had it to do over again I would have gone with the Brison as it is 1.2 lbs lighter and similar thrust.

As to the hatch mods, no, you don't have to but it is easy to do and you will have a much much easier time of it when it comes to field assembly. Those springs in front of the wing tube are a bear to get to if your hands are as clumsy as mine.