Call Bayshore Marine in Annapolis for referral.
Most standard 'automotive machinists' can and do do valve work on diesels ... the profiles and standards for all the 'tapers', face/seat angles, etc . on Yanmar valve systems are all published and easy to follow, and easily accomplished by a competent automotive machinist.
If the engine or head is 'out' and you take it to the machinist ... will probably be the 'most economical .... but you will have to know how to correctly reassemble, etc. to the block.
Get the Yanmar 1GM 'SHOP' or SERVICE manual.