Not really sure whether it contains a normal ATX psu or an SFX psu, think ATX looking at it, with that said would an SFX psu with ATX bracket maybe a solution for such a case. Usually those are modular and are smaller. They likely will cost more though.
A vid about the case so maybe some help with changin the psu.
For options,
or other wattages from these.
For a cheaper ATX psu something like this one
https://pcpartpicker.com/product/3F...onze-certified-atx-power-supply-cp-9020122-na
but comes with all cables attached to the psu. Saw a remark in one review/overview where they said that using a modular ATX psu might give problems with connectors and available space. Kinda why i suggest the SFX psu with ATX bracket.
All said i don't know the system so what really works in the end is hard to say. Also keep this in mind,
max 290 mm ATX Graphics Capability
Guess you can get that 980 ti for little money, might be better options like a gtx 1070 which use less power and likely are abit smaler as well.