Should I replace the PSU and motherboard in this new prebuilt?

If it is running properly, I would leave it alone for now. You paid for a complete system with a two-year warranty, so contact the seller if it develops problems rather than immediately paying to replace working parts.

Check the warranty terms before changing anything. A replacement may affect coverage, but the exact terms depend on the seller and your location. Also, a warranty can mean repair time and inconvenience, so it is not quite the same as preventing a failure.

Gold efficiency and 750 watts do not tell you everything about PSU quality, but non-modular cables do not make it unsafe either. The model was described as a mid-tier unit in one assessment, which is not enough on its own to prove it is dangerous or needs replacing immediately.

If you want more confidence, look for a detailed review of the exact 750AE model rather than judging by the brand alone. You can reconsider a replacement after the warranty period, or sooner if you find credible evidence of a problem.

A small VRM heatsink is not proof that the motherboard cannot handle the 7900X. It is a reason to check how the system behaves under a sustained CPU workload, though. Watch CPU temperatures and performance, and check motherboard or VRM temperatures if the board exposes them.

If it overheats, throttles badly or becomes unstable, document what you see and ask the seller to investigate. The motherboard does not need replacing just because it looks basic.

If you are unhappy with the component choices and still have a return window, compare returning the whole system with the cost of keeping it. Replacing a motherboard and PSU in a brand-new prebuilt adds expense and work, and building your own instead may cost less, though that depends on current prices.