Here are some links for TZ throttles and parts. These are excellent quick and longer pull two cable throttles for bigger Dellorto or Mikuni type carbs. May have too long a pull for regular Amals.
Genuine Yamaha parts, except the throttle tube which is no longer available. Note: Fondseca used to offer a replica tube separately, may still. http://www.partzilla.com/parts/search/Yamaha/Motorcycle/1982/TZ250J/HANDLEBAR - CABLE/parts.html#
Fondseca replica throttle assembly. http://www.fondseca.com/index.php?main_page=product_info&products_id=1467
Domino replica assembly. https://revivalcycles.com/products/domino-dual-throttle-pull
Genuine Yamaha parts, except the throttle tube which is no longer available. Note: Fondseca used to offer a replica tube separately, may still. http://www.partzilla.com/parts/search/Yamaha/Motorcycle/1982/TZ250J/HANDLEBAR - CABLE/parts.html#
Fondseca replica throttle assembly. http://www.fondseca.com/index.php?main_page=product_info&products_id=1467
Domino replica assembly. https://revivalcycles.com/products/domino-dual-throttle-pull