i've worked on many over the years. a few of the e21 m20 specific parts are a little more difficult to locate these days (namely, the timing belt, at one point in time, unsure today), but are largely still available.
if one felt like, you could convert the timing components over to e30 325i parts (which were shared between m20 powered e28 and e34 cars), and if you really felt like it, you could install the Motronic harness and controller, etc, off the same e30 325i. in my opinion, it would be easier to track down parts for those cars, since they made a lot more of them. of course, at that point, one could make the argument for installing the whole 2.5 liter engine, but that's up to you.
either way, the e21 323i are really a special car. congrats on your newest acquisition. post some pictures!
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