Hey everyone builds going smooth again

Got the fuselage all rubberband together, not gluing yet though
got a question!
I do not have an engine yet, so can I install the engine and everything AFTER I have the build completed? or do I have to keep the F1(firewall) former out of the build?
OR would it be easier to just take the OS 46 off my Nexstar and mount all the screws in accordance to that? are all OS 46's the same size... becuase I know the engine on the Nexstar isn't OS's performance engine, and I want to get a differenet type of OS engine.
also are all 46 Glow engines the same size? if thats the case I will just mount the hardware via the Nexstars engine
heres some pictures