Even though you are at the heart of where much of the development was done, and where at least some of the code is maintained, it might be difficult to find a consultant that is still proficient in 500 specifically in the Irvine area. Also, the UK generally required a lot of unusual configuration of the overall functionality and modifications were typically needed for processes and reports (centered mostly around taxes) that aren't required in the US. 

I would suggest you start by determining who the reseller of record is for the UK subsidiary, then work down to potential consultants or a new ROR if the reseller no longer exists or works with Sage 500.

Even though you are at the heart of where much of the development was done, and where at least some of the code is maintained, it might be difficult to find a consultant that is still proficient in 500 specifically in the Irvine area. Also, the UK generally required a lot of unusual configuration of the overall functionality and modifications were typically needed for processes and reports (centered mostly around taxes) that aren't required in the US. 

I would suggest you start by determining who the reseller of record is for the UK subsidiary, then work down to potential consultants or a new ROR if the reseller no longer exists or works with Sage 500.