Motherboard repair for the Amiga would be a good start. Often the boards fail to boot or show distorted graphics with little or no reason to the average user. Some people offer a 'Caps' replacement service but very few offer a multi pin smd replacement or missing solder pad/broken track repair. (if anyone does, apologies, I have never found you).
Would you be able to offer 'one off' add on boards like an internal speaker board for the Amiga 600?