Randy,
You can make homemade syrup with this instructions. Honey can be used as a substitute for golden syrup too.Lye water sometimes called as alkaline salt is an alkaline solution. Tradition Chinese lye water is made with Kansui powder (蓬灰) and alkaline. But today’s version is a combined alkaline solution contains potassium carbonate and sodium carbonate. The ingredient label on the store bought bottle contains 80% pure water, 15% sodium carbonate and 5% potassium carbonate. The lye water can raise the alkalinity (pH) to neutralize the acid in the golden syrup. Baking soda does too, but sodium hydroxide is far more potent. Another purpose is the Maillard reaction, which is responsible for the crisping and browning crust skins.
This one might quite hard to find. Lye water You can try to search it at large Asian store, especially with lots of bakery ingredients. There are several approaches to make substitutes for lye water used in mooncake. They might work slightly different but can yield a very similar result.
Approach 1: dietary alkali powder with clean water at the ratio of 1:4.
Approach 2: If you use baking soda directly, you will get much softer and less browned crust. So firstly bake baking soda on a lined baking tray at 120 degree C for around 1 hour to turn it into stronger alkali. Do not touch it during the process to prevent skin irritation. And then mix 1 teaspoon of baked soda with 4 teaspoons of water.
Approach 3: A better substitute than baking soda due to its high pH is sodium carbonate. The pH value is between the pH of baking soda and lye water. I find this one on amazon.

So a happy Mid-autumn day to you, too!

I use lots of my daughter’s favorite in tis batch including walnuts, chestnut, sunflower seeds, white sesame seeds, cranberry, raisin, almond, pecans.