Hey, I'm not in to paying so much for the new battery for my wheelchair. Any alternatives?

Yes you can. Although it is not as good. Problem with internal structure of the car battery. It is meant to crank alternator for very short time - only a few seconds.
Here you need long stamina battery instead of fast energy burster. Sealed lead acids are the best for you as they have better design for extended time powered applications. Also keep in mind, not every gel cell (sealed lead acid) is the same. We have all sorts of sealed lead acids designed for different application.
This is not a sales pitch, but the reality. I tried car battery on my electric bike and had them running for a while. But they were much heavier, bulky and not the same big capacity as the one I found as alternative.
I actually like the LFP (Iron Phosphate batteries). But they are kinda expensive....