I'm no expert, but did do some research on acrylic tanks before I bought a used Seaclear System II 50g recently.
Acrylic is definitely a lot stronger than glass and will bend/bow in situations where glass could shatter. Looking at the 'cutout' the previous owner made I would say that is the reason for the bowing, he basically removed nearly half of the cross-bracing. I have no idea how much it would cost but the ideal solution IMO would be to have the top replaced by the original manufacturer and then switch to a cannister or sump instead of the hob.
If replacing the top isn't feasible then I would want to do something about the cutout itself - the corners should be rounded off, not 90 degrees, and especially not left with those 'overcuts' as they are the places where cracks are going to start if the strain does become too much. You could maybe 'sandwich' the cutout area with two pieces of acrylic cut with rounded inside-corners and then bonded to the top (clamped tightly to be sure the whole assembly is straight and level) making sure the rear edge of the two pieces bond to the back of the tank as well as the top - even better would be to get rid of the hob and use two pieces of acrylic an inch larger than the hole and another piece filling the hole as tightly as possible, might not be as strong as a replacement top piece (or as tidy) but would relieve the strain on the rest of the top.
Whatever you do, if you don't replace the top completely I would keep a close eye on the top rear seam for any signs of cracking around it (don't know if acrylic is the same, this but I was taught that when a seam is welded between two pieces of steel it's not the weld that gives it's the steel right next to it - but that was 30+ years ago).
Don't know how much you paid for the tank, but knowing the cost of new 90g acrylic tanks it's worth spending a bit more to make it secure even if it's just to have 'peace of mind'.
