Well, the GP7/9 were identical externally, and the GP18 had a different screen on the radiator, and the GP30 shared the same chassis as the GP20, and I think the GP30 also shares the frame of the gP9, which shares the gP18, so yeah, GP7/8/9/18/20/30 might all fit. Don’t act soley on my word though, I ain’t an expert or nuthin’, just my [2c]
You might also put an old Atlas GP7 shell on it, a little filing of the shell may be needed. I did this back before Life-Like came out with their GP7’s and GP9’s. I had a P2K GP18 that took a trip to the floor and found the Atlas shell at one of Howard Zane’s train shows.
You can go on HO Yard Sale on Yahoo and post a request for a shell when you figure out which one you want. I see a lot of undecorated shells up for sale there.