MarkT that was just the kind of thing I was thinking of, do you have any model references for that part so that I could look for one on the local used parts sites. It's often difficult for me as some things listed in Japanese don't aways turn up with the word that you think it is (if that makes any sense) cheers for the info

You'll want to look for automotive lamps. 

The USA had very strict headlight laws for decades...  originally every car had to have two 7" diameter round headlights...  eventually they allowed four 5 3/4" round headlights...  two were hi/lo and two were high beam only. 

Since the US was such a large car market, many manufacturers around the world used these round headlights so their cars could be sold here.

What you need is a 5 3/4" round headlight that takes an H4 bulb.  Late 1960's and 1970's Japanese cars with 4 round headlights came with these. The old Datsun 510 (aka Bluebird) came with four 5 3/4" headlights as one example.

There may be newer cars/trucks that came with the four 5 3/4" round glass headlights

I've used a 5 3/4" H4 headlight lens/reflector from a modern Harley Davidson with a 6v 35w/35w bulb. 

Koito is a Japanese brand that made 5 3/4" H4's and still might? 

Amazon and ebay have many available here but not sure how that works for you in Japan

MarkT is correct on the Harley headlight as I have one on my Dyna 

so on my DT175B the head light is very dim while idling it is at 6.5v or higher but for light output it is very weak. So once you find a bulb set up that works for you could you let me know what you end up using? I was wanting to go with a replacement bulb versus the whole headlight.

Dogboy, mine is very weak at idle too, probably lights up with revs but I only check by putting my hand in the front, so not much of a test. I suppose the best way would be to find a reflector that had a fitment for an H4 and then to use a H4 LED for best results (like riding in the dark) but I paid silly money for a bulb just to be 'street legal' and get the job done. I fabbed up a 'boot' with some material (hope it doesnt burn in there.....) but should really pick up a new rubber holder, hope that you get your light issues solved too