Newb question for seasoned custom bike builder

Hi all, I purchased a vintage motorcycle (1963). I don't know anything about motorcycles but wanted to learn with it. Plan is to make it into a cafe racer, but I want to swap the front fork for something newer. I have a motorcycle friend said all I need really is my dimensions for the neck of the bike.​Them being 170mm in length and 30mm ID.​Question is, I've seen a build with my bike using EBR forks (because apparently vintage moped parts fit on it and are apparently the only aftermarket forks that do). That being said how small can I go on the head tube? Would I need to find a 28mm or could I use a 26mm?​The only forks for 28mm I see have a length of 180mm, which I feel like would be too short to bolt down once you use the top of the fork set. Am I wrong? I'd ideally like to get as close to a bolt on as possible, but if I have to take it somewhere to get the old neck welded to the new fork set I'll do that instead. via /r/CafeRacers https://www.reddit.com/r/CafeRacers/comments/ga1fu2/newb_question_for_seasoned_custom_bike_builder/?utm_source=ifttt