Many UPS batteries are designed to have a long , low amperage draw down.. They do not usually have the high discharge ability of starting batteries and are unsuitable for quads or bikes. Make sure if you go that route to check the CCA rating on them if you go that route . The batteries in the larger units would likely be fine if their dimensions fit the battery box. I'd suggest getting one of the new Lithium ion jump starters to take in your emergency kit on your quad. They are compact, lightweight and have the power to boost a car. They're a bit pricey , but handy to have for power peace of mind if you're out alone . Many of those jump starters also have a 5 v outlet that can be used to recharge a cell phone or any other accessory that uses USB recharging or operation.

The alternator will power all the electronics and recharge the battery . Yes, cold weather will make the quad draw more on starting cold. The CCA is the Cold Cranking Amps rating. it is less than what is available when warm. I searched on the 3 batteries . and found that yes, there is a 4 AH difference rather than 2 as I assumed from their part numbers . You'll notice the OEM battery has a 16 in its part number, but actually an 18 AH rating while the cheaper one with the 16 in its id is actually only 14 AH The expensive one has a 20 , but only 18 AH rating .. Manufacturers playing confusing games with their part numbers. The BS in the battery id means it is a sealed battery. About the only difference to me in the AH capacity of the batteries is in how long it will crank over the engine before it is depleted. Once running , the alternator will run the quad and recharge the battery. No quad maker makes one where the charging capacity is less than the power draw. To me , the only rating that makes much difference is the CCA. That is the one that determines whether it will start the motor in the cold

If the batteries are physically the same size , the loss of 2 Ah shouldn't make a lot of difference. More important is the cranking amps available for starting.