Why ⅛” chain width?
 

I’m working with some old track gear now. I’m building up with a ⅛ chain, sprocket, and chainring, on a Stronglight 49D chain set, Additional chainrings are getting scarce, so I’d like to use 3/32”CW with the 1/8” chain. Does this raise any concerns?

I was about to say hold up plenty of 1/8 chainrings out there but then decided let me make sure that the crank isn't something French like 50.4 and of course it is 50.4. You should be fine with a 1/8 chain on 3/32 parts you just cannot do the opposite if I am remembering correctly.

However you might consider getting a different crank with a 144 BCD so you can run standard track chainrings and find loads of options very easily. I am sure in the C&V section or elsewhere on the internet tubes you can find someone who wants a Stronglight crankset or there might be an adapter out there still. I believe SunXCD made one that could at least get you to 130bcd and you have loads of options. You can also just continue as normal but having more choices is kind of nice. I have an old Campy 116 bcd crank that I was going to use on a 1x build and I am thanking myself I didn't and found a nice Tiagra HTII crank so I can find more chainring options and easy install.

Not in my experience. 1/8" chain runs fine on both 1/8" and 3/32" drivetrain components, allowing you to mix and match as needed. TA did make track chainrings for 1/8" chain that can mount on the 49D crank, but they're pretty scarce these days. They can be distinguished from the road chainrings by a "3" stamp, indicating 3mm (1/8") chain tooth profile, just below the tooth count stamp.
https://cimg0.ibsrv.net/gimg/bikefor...2ed86a4d78.jpg

I see. I actually have one in 49T, but I’d like a 45 or 46. It’s a Sturmey Archer bike, and I have an FM hub for it and a good AW (the original pick). I bought the FM as an enhancement but it needs work. So I’ll first build wtth the AW, geared 46-17. Then if the FM gets fixed I’ll install the 49, to compensate for the lower step up gear of the FM, 1.125 versus 1.333 offered by the AW. The original chain is sized for the AW.

I guess to be correct I should have said that I’m working with some track-like gear. Sorry about that, folks!

reason for not going with a modern 1x chain set is first, they looked sorta pricy, I have a few 50.4 crankarm sets on hand with spindles which will drop right in to my original Olde English bb cups. So far it looks like I’ll have very low Q with no chainset to frame interference.

Peter White carrys 5-pin TA rings that will mount on the Stronglight 49D crank. They're "outer" rings, 3/32" width, for the Pro-5 "Cyclotouriste" crank, but as noted above, they will work with 1/8" as well:
TAPL-50080302-40 TA Pro 5 Vis outer 40 $48.00
TAPL-50080302-42 TA Pro 5 Vis outer 42 $48.00
TAPL-50080302-44 TA Pro 5 Vis outer 44 $48.00
TAPL-50080302-46 TA Pro 5 Vis outer 46 $48.00
TAPL-50080302-48 TA Pro 5 Vis outer 48 $48.00
TAPL-50080302-50 TA Pro 5 Vis outer 50 $54.00
TAPL-50080302-52 TA Pro 5 Vis outer 52 $54.00
https://www.peterwhitecycles.com/tach.php#pro5