https://github.com/emard/ulx3s that's an open design. https://www.digikey.co.uk/product-detail/en/alliance-memory-inc/AS4C64M16D1-6TCN/1450-1332-ND/5797372 found a compatible (TSSOP-66) ASIC with 128 megabytes of RAM. which is probably good enough. l.