Definitely simpler and quicker to keep the alternator and would usually be argued that this is an improvement too.
I have done this in reverse and fitted an alternator to a 499cc engine by transferring the cowlings from the 652cc engine. So you could achieve your requirement by putting those from the 499 on the 652.
The alternator has an integral mounting plinth and fixes to the crankcase using setscrews. The dynamo has a separate mounting-cradle which is secured using nuts on studs into the crankcase. But all alignments and geometry were the same in my experience; the job was no more mind-boggling than anything else I've encountered on a 500; the alternator fits over the studs so the dynamo-cradle should fit using the setscrews.
I found it simpler in the end, regardless of which permutation of cowlings is used, to fit the heater-duct designed for the 500R/126 in place of the original; this avoids undesirable contact with the starter-terminal. You also need to fit a 126 starter-pull-cable and obtain a small, nylon cable-stop which fits underneath the heater-duct. Ideally you can also replace the heat-shield above the exhaust with the one off the 500R. There was some disagreement with me but I prefer the 499 rocker-cover on the 652 engine when fitted in a standard 500 because the HT-lead mounting bracket makes the leads abrade on the engine-lid. You also need the longer studs into the timing-covet for the engine-mounting.
I discovered most of the above by trial and error but I benefitted from a lot of guidance from some very helpful contributors here.
I would be very cautious about trusting an unknown engine, on an important journey, to give you reliability after the various modifications that will be needed. Ideally you would need quite a few miles of shakedown to get everything running just right.