I'm assuming you're talking about the older, pre-1965 reels. I have a bunch of trout-sized JW Youngs. Hardy pawls will fit them and probably yours too. The plastic spool release caps on the older reels crack very often and will therefore be very hard to find. When I worked at Harkley & Haywood back in the early 1970's, these reels were still much in use and our reel repair guy used to machine aluminum replacements at a very cheap price. Your best bet would be to find a machinist.
You can take your chances and give Garry a shout at www.milltackle.co.uk
Garry worked at Youngs and now has all the remaining stock. He is notoriously hard to get a response from but i have had a few and he has been somewhat helpful when i do finally get through. The bakelite latch covers mentioned above are virtually impossible to find as i have been after them for some time. Garry does have replacement plastic covers but they really cheapen a reel(imho),, but take what you can get(they used to replace these free of charge,, but not any more). He also has springs and pawls available as of three weeks ago as thats the letest communication with him. Original 3" reel feet are also a problem but if you are looking for square line guards then search ebay as there are several to be had in an auction there. I'll PM you with a gentlemans email address as he is the most knowledgable man on young reels you will find and if he doesn't have the parts then most likely will know where you can get them.
Hope this helps.
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