charging 12v lead acid battery with OEM charger
That charger should be fine for a 7 Ah battery.

The maximum recommended charge rate for a lead-acid battery is 1/4 of the Ah capacity, which would be 1.75 amps for your battery.  The cvharger you suggest is rated to provide 0.75 Amp, which is well under the maximum for your battery, but sufficient to charge it in a reasonable time - about 10 hours for a full charge.