The GTX 745 is an OEM card, so it's a little tough to judge where it 'sits' performance-wise (and therefore, what would be a worthwhile upgrade) but, from what I can find/judge/guess, it sits somewhere in the GT730-GT740 space.
Looking at a couple of videos, there appears to be a decent amount of physical space for a GPU:https://www.youtube.com/watch?v=2dpa4vb0PW8
But the power options are obviously a stumbling block.
The GTX745 is a ~55W TDP card. Anything worthwhile upgrading is going to be a minimum 75W TDP, if not more (a GTX 1060, for example is 120W).
It does appear to use standard PSU connectors though - so a replacement PSU is an option BUT there's nowhere to put a "normal" PSU in there - so you'd need to migrate to a new case.https://www.youtube.com/watch?v=-RQ-IW8fIqs