Usually the "forbidden territory" about PSU's is only about the fact that they are (were) connected to mains, if you give the PSU the time to let the capacitors

Usually the "forbidden territory" about PSU's is only about the fact that they are (were) connected to mains, if you give the PSU the time to let the capacitors discharge (a few minutes in the worst cases) they are perfectly safe. What I would do would be to wash it with distilled (or demineralized) water first. If the gunk goes away, there is no need to use the isopropyl alcohol at all, but otherwise a quick immersion in isopropyl is also ok. Both with distilled water and isopropyl alcohol, I would find a suitably sized container and put in it just the right amount of liquid to cover the part(s) where the gunk is visible (the top right connector in your case). Distilled water won't do any damage and you can leave the PCB in it as long as you want (to give it the time to moisten/dissolve the sugary gunk), with isopropyl alcohol there won't be any damage to the actual components, but if you keep it for long you may well have some esthetic damage (like discoloration of some plastic). There should be isopropyl alcohol available in spray cans too, I wouldn't go for contact cleaners if not for the actual connector pins, after they are cleaned, at least the types I am familiar with do leave a (protective) residue that is good on the actual connectors, but that you don't want elsewhere on the board.