It depends on how much work you want to do to it. Any of the SOHC series (1100-basically a Yugo motor anyway, 1300-Early x1-9/128, or 1500-Late x1/9/strada) will fit with a few mods such as the oilpan and intake for the correct (same as 128) mounting angle. The 1300 actually had a little more power than a stock (US spec) 1500 due to a larger carb. Of course if you are going to put an aftermarket intake, cams pistons, etc, then the larger displacement 1500 would be the route to go. It would also mate up nicely with a Strada 5 speed if you want OD.
I know you wanted a carb, probably due to the simplicity, but the FI 1500 out of a late x1/9 would be really slick. I've got one in my x1/9 and its noticibly faster than a stock carbed 1500. My Yugo is still a stock 1100, but I did add a Strada 1500 carb which is a straight bolt-on except for the air cleaner and GREATLY improved drivability if not outright performance. It also has a built-in idle control for the air conditioning which works much better than the silly dashpot on the accelerator pedal that Fridgette used for the dealer Air.
Al