I have seen very few bad examples of IC1. It seems to be a pretty robust IC. There is 1 case where the owner seemed convinced that the IC was bad but because I did not personally check do the testing, I can not confirm this to be the case. This deck has lots of failsafes. However, with all electronic components, it's nevertheless possible for it to go bad.
If it is indeed bad, then that is bad news because IC1 is not available from any source. It is obsolete and as it was used exclusively in the M90 and no other models that I'm aware of, there is no alternatives either since it isn't worthwhile for anybody to manufacture them beyond keeping the M90's alive (and once again, this IC does not fail often). Therefore, the used market is the only source but getting someone to part out an M90 is not easy. However. Member Mike here has recently replaced his deck with one he found off eBay. I'm sure his original deck could be salvaged but his tech wanted to do the swap route rather than the fix route. If his deck is still available, it might be worth taking a look at. Search for the archives and you'll find his thread where maybe you can PM him.
As for the IC itself, remember that the inputs need to be pulled low to trigger. Also did you do voltage checks to ensure that the IC is getting power? If it is not, it obviously will not work and you should investigate the respective power regulator for that 5V rail. Although the fusible resistors protect the regulators, just because they aren't blown doesn't mean either that transistor or zener or the resistor has not gone bad, which is expecially true if one of the resistors had been replaced in the past with non-fusibles since the fusible resistors are a bit hard to find nowadays. Also, you should not presume it is getting ground either. If you tested for voltage with the black meter probe at a ground other than the one the IC sees, the meter may show 5V but if the IC itself does not see ground, then it likewise has no power.