hack checks for Gen 1/2 are pretty minimal, especially when it comes to eggs, because there aren't much datapoints nor correlated stats. If you look at the stats of the Ditto and Haunter, it may be possible to determine via DV inheritance whether the Gastly is indeed real.
try the guide with PokeFinder. If you can't follow the steps to fix them then try using encounter database to generate a mon, and not change any details.
Im gonna explain jksv (wich its the one i use)
Open jksv
Select your profile
Select pokemon SV
make a new save
Send the new save to pc
Open file "main" on pkhex
Edit to your liking
Send back folder to SD
open jksv and select pokemon
Hold Y to restore save