Is the PG-12V200FR an exact copy of the MK 8G8D?

No, and the differences matter before you order. The Power-Sonic is AGM where the 8G8D is gel, its case runs 20.55 by 9.45 by 8.58 inches against the MK's 20.44 by 10.97 by 9.98, and it connects through 8 mm threaded studs rather than SAE posts. It covers the same 8D-class duty at 226 Ah and weighs 13 lbs less. Where the tray has room and the charger suits AGM, it serves; where the spec demands the original gel unit, buy the 8G8D itself.

My charger is set for gel. Does that change with this battery?

Check it before the swap. Gel and AGM want different charge profiles, and a controller left on a gel setting will undercharge an AGM over time while an aggressive AGM profile can damage a gel battery going the other way. Most quality chargers and inverter-chargers switch profiles in settings. Send your charger or controller model with the quote request and the specialist confirms the right profile in writing.

What does the 1,200 A rating cover?

Ten seconds of maximum discharge, which is a burst figure for motor starts, inverter surge and fault clearing rather than anything continuous. It reflects the roughly 3 milliohm internal resistance of a large AGM block. Runtime still comes from the amp-hour ratings, so size the bank from the 20-hour figure and treat the 1,200 A as headroom, not capacity.