The carburettor model is the same so most internal parts should be the same. The needle is probally the same but I think the jets (seats) should be smaller. Float valve, float seat and gasgets is available from most dealers but jets and needles is more difficult. Try the YAMAHA dealer, they still have som parts for the Hitachi carbs. 4X7-14925-24and 4X7-14925-26 is the parts no for the main jets for TR1 (size #124  #126. The needle: 4X7-14918-20.
Some links;
http://www.cyclewareables.com/pages/hitachi_main_jets/hitachi_main_jets.htm
http://www.motorcyclecarbs.com/
http://www.partsnmoreonline.com/

I need to replace the needle valves and thier seats (need float chamber gaskets too)on my Hitachi carbed 1985 998cc TR1, any idea where i may obtain them or maybe a pair of used carbs?
cheers.