UI6 produces the missing signal, and also the main clock to the 68030. Either it is faulty, or missing an input signal, or the output is shorted (possibly by corrosion, but it is far from the capacitors). A scope or even better a logic analyzer would tell the story. The solder pads being dull but no serious corrosion makes shorts seem less likely, unless there are some directly under a cap causing unseen evil. I'm feeling like you-capacitor problems are possible here, but a long shot for taking out horizontal sync. Capacitor goo might be taking out some other signals and inhibiting proper booting, however. Especially if corrosion has added or deleted some connections under the SCSI and/or floppy controllers UI12, UJ11.

If you have a parts donor board you could replace UI6. Or there's also a trick test where before replacing, you carefully piggyback a good chip over the suspected bad one and compare outputs. In the case of piggybacking UI6, all pins need to touch except lift all outputs 12 thru 19 inclusive (bend them out to slightly clear or use tape on the lower DIP) and use a scope to see if there is identical output activity on most and in particular look for horizontal sync output on pin 18.