[EDIT: You'll find the QL PSU schematic with voltages annotated here: http://www.dilwyn.me.uk/docs/hardware/QLperip.zip
You'll find quite a lot of relevant info already discussed here on the forum across many good articles, but here's some thoughts gathered together...
Before going to the trouble of replacing ICs that are soldered to the board, the following ICs that rely on the +5v rail are socketed and could be replaced with known-good units more easily:
1. ZX8301 (IC22) - the display and DRAM controller (the so-called 'Master Chip')
2. ZX8302 (IC23) - the peripheral controller - 'PC'
3. 8049 (IC24) - the secondary processor or 'IPC' - managing Serial input, Keyboard and Sound *
4. 2x ROM (ICs 33 & 34)**
5. MC68008 (IC18) - CPU
Items 1..4 are fairly easy to come by (e.g. from Rich's 'SellMyRetro' auction site), although the ZX8301 is getting more scarce...
The CPU is trickier to find - at least in the required DIP form-factor.
[EDIT: Found quite a few 48-DIP form-factor 68008s on eBay at a very reasonable price - make sure you go for the -L or -P suffix - and not the 'FN' suffix which is in PLCC or similar form.]
* Instead of replacing item 3 with another 8049, you'd want to take the chance to upgrade it to the 'Hermes' equivalent.
** Instead of replacing the originally masked-ROM ICs directly, consider a Minerva EPROM upgrade or better-still, Marcel's updated Minerva+QL-SD solution (but only once you've got the rest of the system running again.)
These items are not especially costly individually, but you'd still be looking at 60 - 120 GBP for all five items, assuming you can still find them - and even then, there's no guarantee that other components wouldn't also need replacing (though typically cheaper.)
Except the TV modulator (which wouldn't itself stop the system running), the other ICs relying on the +5v rail are readily available 74-series logic/buffers - though not socketed (there's a couple of RS-232 line drivers, but the system would still function even if they were dead.)
Aside from the ICs, my guess is that the other passive components are probably still OK - most of the smoothing caps in the QL for example are rated at 16V or more, so may well have survived.
Whilst I wouldn't wish to discourage you from pursuing the potential satisfaction of fixing-up this unit, bear in mind that you can pick-up another working QL for less than that from time to time on eBay, etc and even a fabulous Q68 might cost less!
[EDIT: How about this replacement QL for auction on eBay? (nothing to do with me...):https://www.ebay.co.uk/itm/Sinclair-ZX- ... 0282.m3503