Good luck, there's some great stores but parts are impossible to find, like other's have said, it might be easier to get it here.
While your in Akihabara, and you like pizza, this place is killer with craft beers!
https://www.devilcraft.jp/
and a 7 story wonder of vintage cool toys
https://order.mandarake.co.jp/order/?lang=en
Shibuya has a ton of record stores, that might be a better place to look for a tone arm, Nerds, Next, Mother's, Manhattan Records etc.
Up the street from Shibuya is Ebisu, stop at the original Sapporo Beer Factory/Museum for some suds and then hit the coolest little cassette/boombox store in the world, The Waltz. They also have hundreds of old issues of Tape Sound and other 70's magazines that are packed with audio photos and info (in Japanese but the photos are cool to look at).
https://www.sapporobeer.jp/english/brewery/y_museum/
https://waltz.business.site/
Ebisu is pretty nice, plan the day around their "trash day" I think it's a Tuesday? Before 11 am people set their larger trash out on the curb, it's all nicely wrapped up and super-clean. You won't believe what they toss, nice old stereos/radios, travel luggage (because they have no where to store it) and much more, it's better than some flea markets.
Dubby Mad is a legend in the boombox world, he's restored a million boomboxes not sure if he is still open but I have a feeling he knows where to get parts.
https://www.facebook.com/people/Boombox-Japan/100069180170314/
Good luck and have fun, walking is your friend, you'll find more neat stores that way.