My approach usually starts with listening to the track to get a feel for what's going on. Sounds like a transistor type organ with an arpeggio. The real key here is the arpeggio - which is easy to replicate in Montage which makes the sound less important to get 100% spot on. I then do research unless I can readily identify the synth/organ used.

Source: http://thewho.wikia.com/wiki/Baba_O%27Riley

Lowrey Berkshire Deluxe TBO-1 organ using its marimba repeat feature

That's about as good as any of a result from research. Only documentation of the register settings (sounds) would have completed the puzzle. But, again, the sound itself is not as important (IMO) than the arpeggio.

If you're interested - here's what the Lowrey looks like: https://www.youtube.com/watch?v=Rw85dqAMmUU

I would probably start with one of the transistor organs and build a user ARP. There may even be a standard ARP that works - but it often, for me, takes longer to find an ARP rather than just build one.

If building an ARP is a mystery - you would basically record one cycle of the pattern in the Performance recorder then convert this to an ARP using the "Org Note" type (I believe). The two parts (accompaniment vs "lead" ) in this organ part sounds like the ARP pattern is different for one manual vs. the other. One seems to repeat notes jumping to different notes and the other seems to repeat notes not jumping around. This can probably be replicated by splitting up into two different PARTs - each PART with a different ARP. The second PART can probably use the same original pattern. Use "normal" mode for this ARP convert. This will let single-note lines just repeat the rhythmic pattern without the notes jumping. Notes will jump (chord intelligent) if you press down multiple keys.

I'm not going to 100% cover the making-an-ARP subject here. There are multiple tutorials for this.

... and adding - sometimes I'll search around for a cheat. Looking for usually if someone has replicated the sound on "Motif" (since Motif has been around for longer). In that search, I found something here:

Source: https://www.gearslutz.com/board/music-computers/185562-quot-baba-oriley-quot-who.html

I'm not really sure what synth that is.. but if you have access to a Yamaha Motif ES or XS series keyboard/module you can get a patch library called VINTAGE KEYS from Motifator.com - Official website for the Yamaha Motif Family which has a 'Baba O'Reily' patch. That, 'Babe', and 'Jump' are worth the price of the library alone...

So if you wanted something more plug-and-play - you may be able to purchase the "Vintage Keys" set.

... along the path of research I'll often find other useful information. The above thread has more information than quoted that may be helpful to you.

That's not the only link. Just the first. There are more.

http://forum.vintagesynth.com/viewtopic.php?f=1&t=31398

This one mentions a Motif user on Motifator creating a sound using a Clav.

I would, in general, suggest your own searching/discovery/cheating/ground-up/etc. when creating your sounds.

And, I would not personally see the need to purchase a set for this sound. That's just me. When covering tunes, I'm less concern about extreme sound accuracy vs. getting the general idea to match in the general neighborhood.

Thank you!