Just bought a 522 for a screaming deal. Just ordered the HK style conversion kit to help the look. Next I wanna replace that hideous top rail. Is this the piece that should replace it?
https://www.ebay.com/itm/GSG-5-GSG-522-FACTORY-Top-Cap-P-PK-SD-/274227017442
Anyone ever done this conversion?

EDIT: took the rail off and it's the normal top, minus 2 screw holes that I gotta fill. Anyone know the correct size to plug those?

Yes that is the correct part.

There are a few other GSG-5 parts as part of a conversion kit (CDNN and ebay have them) that are still around and have the other HK-ish parts like the front sight and cocking tube assembly for a more HK look
https://www.cdnnsports.com/gsg5-gsg522-dlx-hk-style-kit.html#.XnHgyXJKjIU
Watch a few of the GSG disassembly videos and replace your parts with the GSG-5 parts vs 522.

The top essentially is just sitting there clamped in place between the two halves of the receiver sections.  Easy to do and just be slow and careful and don't lose any parts or springs.

https://www.youtube.com/watch?v=Y7nO_t1sB9M

Just purchased the kit you suggested,  HK push pins, and the screw replacement kit. Now If only I can find someone to create the navy lower from the ICS airsoft receiver.