The best method is to use root cuttings or air-layering. The plant suckers profusely, so division is another option. Softwood cuttings can be taken in spring, but require bottom heat of 75F, use of rooting hormone, and a high humidity to root. Seeds may be planted in autumn, but germination is uncertain. Use a compact soil mix.
Roots tend to be fragile and few, so caution in repotting is necessary.
A deep pot is generally safer than a shallow bonsai pot. When Polyscias are to be root pruned, it helps to induce a short dormancy by keeping them at a temperature of 50F for about three weeks prior to repotting, then returning them to a temperature of at least 59F immediately afterwards to stimulate root growth. The Hearst Garden Guide to Trees and Shrubs.