I’d run it first, adjust the tappets and go from there. A set of springs is about $150 and I’d lap the valves if you’re planning to remove the head. Time to start browsing the Burton Power website!
In the UK
https://hpemotorsport.co.uk
https://www.vulcan-engineering.co.uk
http://www.wilcoxengines.co.uk/xflow.htm
Maybe Tony Ingram. http://www.lotus7.com/Home.html
For sure Burton Power in the UK would have them. I’m sure there are other US suppliers. I don't have any valve seals on mine and never had an issue.
Did you remove the head or just valve cover to check? Probably worth lapping in the valves if the head is off.