CGM Ultimate engine - big mistake?
Go with the YS91FZ it has more power than the 120 you have now. It is probably lighter too.
You shouldnt have to move the firewall any. The planee is designed for a 120. I have seen quite a few of these with 120 saitos and YS 120 without moving the firewall.
Good luck with your choice and enjoy the plane. It is a real good flyer!!!!!!!