Yes, get the mount and use it to mark you firewall. The engine you are going to use isn't that much different, a little heavier though. In fact the present mount might work, check it first.
As to alteration/mods, DON'T get carried away, if this is your first plane best to just build it and FLY it . You'll have plenty of time to ALTER and MODIFY later!!!!!!!!!!!!!!!!
*FLY FOR LIFE*