Holly crap..... Well it looks to me like the tracks on the back of the mod chip must have shorted out something directly underneath it on the motherboard.
Very nice pictures.... quality wise that is..... cuz really those picture made my spin curve a little. I'll have to see the chiropractor today now.
The fuse on the PSU can easily be replaced. But it might not be worth it, cuz looking at the other fried components they might be replaceable but there also looks like there might be damage to the tracks in that area as well. Might be worth it to sell it off as parts (
www.rickspsxparts.com) and start over.
**EDIT**
Or if it didn’t short out from the mod chip....something else obviously did! When you screwed the case together whatever it was got pinched together tight enough to make a connection and short something out. That’s why it didn’t do it when you had the case off of it.
Good luck.