Will the UB-30H drop into the 8G30H's spot?

Dimensionally it goes where the MK came out: 12.91 by 6.77 inches against the 8G30H's 12.93 by 6.74, and 9.29 inches over the terminals against 9.75, so it sits about half an inch lower under the same hold-down. Both connect through bolted ring lugs. Measure nothing but the cable reach to the terminals, since the posts sit fractionally lower, and the physical swap is done with hand tools.

Do my gel charger settings carry over?

The chemistry carries over, but check the numbers. UPG publishes a 14.2 to 14.4 volt cycle-charge window for the UB-30H, which sits inside the wider 13.8 to 14.6 window the MK gel line uses, so most gel-profiled chargers serve both without adjustment. Confirm your charger's gel setting lands in that band and the replacement is electrically settled; send the charger model with your quote request if you want it confirmed in writing.

The MK is rated 97.6 Ah and this says 100 Ah. Is it a bigger battery?

Treat them as the same class. Manufacturers rate 20-hour capacity against their own test cutoffs, and a 2.4 Ah difference between two makers' 100 Ah class gel blocks comes down to rating methodology rather than any meaningful runtime gap. Size the application the way you did for the original and the UB-30H carries the same duty.