The A-1 (like the AE-1) uses a PX-28 battery. The PX-28 is a currently available battery. There are a number of versions available - alkaline, silver oxide and, I believe, lithium. As I understand it, the Canon A series cameras will function equally well with the alkaline and silver oxide variants, with the silver oxide version lasting much longer. The lithium version didn't exist when the A-1 was being manufactured, so I don't know whether they are compatible.

When you refer to your A-1 shots being "brighter" I'm assuming that you are referring to prints or scans. The differences between the cameras may be due to your metering technique and/or the differences between the metering sensitivity patterns for the cameras (e.g. the Pentax may be metering more of the sky than the A-1). Alternatively, one or both of the cameras may be out of calibration.

You can adjust the exposure by adjusting the ISO (ASA?) setting on the cameras. To make your prints darker on the Canon, increase the ASA setting. You will need to experiment to find the right amount, but if you wish to use the Pentax as your target, I'd suggest using both cameras to take reading off something like a blank wall illuminated by the sky on a cloudy day, and then adjust the ASA on the Canon to make those readings match. Then note how many "stops" you have adjusted the meter for.