I'm not 100% sure, but I doubt that this old AS toilet is a low flow design. Even if it a low flow, it would be one of the very early models which were mostly dogs. (All makes) It may be time for you to consider a new toilet. I do think you are correct in assuming you will have to tear into the wall. Probably a fairly major job, maybe more than DIY, and this would make a new low flow toilet even more sensible. I'd suggest looking at Toto toilets.