I don't see a kit based on the 61 family. Everything I have found so far says that to create a custom board, first start with a kit in the same family then alter the BSP. Can I alter a BSP for a 62xx-family MCU to use a 61xx-family part, and are the instructions the same or different?
Yes, you can select a PSoC 62 device BSP and then modify it to use a 61xx family part. Please follow the instructions present in Creating a BSP for Your Board in the BSP User Guide (shown in the image below) to create your BSP.
Rakshith M B