One such HKS engine that I know of in the UK has been experiencing problems with loss of compression in one cylinder and I'm told that valve seats receded into the alloy cylinder head. I'm also told that this type of engine doesn't have a happy track record in the UK, but after the manufacturer discontinued its production, there was an outcry in the USA where it is much better liked. Production has resumed after the introduction of modified cylinder heads, so I'm told.
https://en.wikipedia.org/wiki/HKS_700E