Not worth it to go after market:
https://www.onlinemetals.com/en/buy/aluminum-round-tubeYou're $150+ bucks for tube, for just the tiller connector (cross bar), and that's BEFORE machining and anodizing. Aluminum tube has gotten expensive. You won't anodize yourself (most likely), so that means painting to keep it from rotting away immediately.
Looks like there's only a certain few alloys of that size, in particular 2024, in only a T-3 temper. Not the best choice and a little softer than what you'd typically want. I, personally would either go new or used original and I'm one to rig stuff up from the sheer joy of "doing it myself".