Egon pretty much nailed it, as did several others. Tools are geared for specific blade diameters to deliver proper torque and 'surface speeds'. Adapting an arbor and trying to achieve proper blade balance at more than design rpm (x2?) would be a nightmare. That said, it rarely hurts to go 1/2 - 1" smaller than what clears a guard. A HFT 14" metal chop saw might be ok with the 14" diamond blade, for ~$100, and would be much safer than spinning it too fast with not enough torque and no guard. I don't see anything mentioned above cutting a curve, maybe a 'faceted' approximation done in steps. btw: You shouldn't
have to cut all the way thru' everything. An angle grinder with masonry blade can smooth bumps/edges.
Straight cuts can usually be 'scored and snapped', as said above. A pneumatic chipping hammer can be used to clean up some cuts with the right point/chisel and right air pressure (burp speed). I grooved a garage floor with a circular saw & diamond blade while everyone stood by yakking. Holding the saw in one hand left one free to spritz the cut as I went with a trigger spray bottle. A trickle from the garden hose would have been easier, but beers had already been passed out & I was on my own. Blade life depends on keeping the edge cool. Dry cuts can heat the edge to where the solder/braze of the diamond particle coating lets 'em fly off, like bullets too, and bare steel won't cut squat.
If I rented/borrowed anything it might be a tub (wet) saw to use with my own blade, depending on the variety of cuts planned. btw: Planning for a job to be 'cake' won't prepare you very well for inevitable snags. Expect the worst, and you'd be ok if things go better than you thought. Taking your time will let what you've learned from the first few cuts sink in and the rest will get easier as you go, as long as the blade holds up. Never too soon for pics, once you get going.
