Terry front wheel (ISO 520 vs 540), big difference? Preamble
I'm building up an old early 80s Terry Precision bicycle. Most of my time has been spent researching and sourcing the front wheel. It's a smaller frame which uses a smaller front wheel for shorter women. I learned that many small Terry frames use an ISO 520 24" front wheel so I went hunting around for that. None of my local LBS could source any 520 parts. I found a couple of online retailers that had the 24" road rims (i.e. not 24" BMX rims, typically ISO 507) and tires/tubes. I bought two rims, two tires, and three tubes for future spares.

I built the wheel recently and noticed that the brake reach on the front was about 20 mm more than on the rear wheel, which is a standard 700C (ISO 622). I realize now that the earliest Terry Precision bikes used the even-less-common 600A wheel size, or ISO 540.

Question
Now I'm in a strange situation where I have invested quite a bit into a slightly wrong wheel size. At worst it will sit 20 mm lower in the front and I'll need to find a longer reach caliper brake for the front rim. Is that a problem?

Side question: does anyone know any companies that sell ISO 540 road rims (and tires/tubes)?

This whole front wheel situation kinda rags on my otherwise purist Terry restoration so I'm not sure what the best course of action is to proceed.

Edit
Turns out that ISO 540 is the size of some wheelchair rims today (along with 507). Has anyone ever used a wheelchair rim for their bike? Where can I find one?

P.S. I know these are many questions and that's against SE guidelines, but to be clear, the main question I have is whether or not using my ISO 520 wheel would be a big deal.
Interesting—Sheldon Brown's tire-sizing chart refers to the 540 size as "British juvenile or wheelchair"

In any case: The 520 wheel (which his chart lists as the Terry size—looking at the Terry website, I see they sell both sizes) will pitch you forward slightly and give you a slightly steeper effective head-tube angle. I don't think that would be a big difference. And you'll need to adjust your brake pads. In theory the size difference could be offset by using a fatter tire, but in practice, I think fatter tires are not made for the 520 size.

I've never heard of anyone using a wheelchair rim on a bike—the only possible downside I would imagine might be A) if they aren't available in the spoke count you want, or B) the braking surface. 

You might contact Terry directly and see what they think.