I'm sure you understand what i said--so ignore me lol-- but just incase--the hole is about 5mm at a guess & it goes vertical from the port outlet to the decomp valve. Just turn the cylinder up & you'll see it. No need to reply, i do go on sometimes.

Thanks everyone for the replies!
I've ordered the stuff and I should put the bike together during May once I'm back from taking care of a farm in France.

I found a relic in France:


Looking forward putting everything together and running it!

I've a question regarding the exhaust: the expansion chamber has some broken parts inside, I've already cut a port, removed the parts, welded it back and...more parts were detached. 
Should I remove everything or anyone has a different suggestion? 
The port I've cut was in a not visible part of the exhaust, for this new rattling part I've would have to cut just above the head in a visible part.

You did well finding those decompressor parts. Is there an Oring for the valve & a copper washer for the main valve body in those parts.

No, those parts are missing: the o-ring is available at cmsnl, the copper washer came with the engine gasket kit.
The small axle pin with splinter is also missing and not available anymore, it will be easy to find an alternative.

I have a question about model differences: I've seen that in 1975 two model of DT400 were made, B and J, respectively for US/CA and for Japan.
Does anyone know if the bike have diffrences?
I 've seen they are marked with different code number for the parts.

"I have a question about model differences: I've seen that in 1975 two model of DT400 were made, B and J, respectively for US/CA and for Japan.
Does anyone know if the bike have diffrences?
I 've seen they are marked with different code number for the parts."

Wiring, including lights, will likely be different, as will the seat. Probably a few other ones as well.

There are quite often photos on the internet, including of brochures, of the Japanese models.

Thank you GR8uncleAl!

I was told this was a B model bike by my dad, now things didn't add up:
The headlight bulb is a fully seamed assembly, the ignition key connector is different than the one of the US bikes, etc.

I don't have the bike here but on the pictures I can see the engine starts with 501, so it is a Japan market bike!

If it is a genuine, non messed about with JDM (Japanese Domestic Market) bike, then it may well have a big red warning light on top of the headlight shell, or on the dash somewhere. These lights had to be fitted by law, and came on when the bike achieved a certain speed - basically a warning light. I've read that cars had an audible warning when the particular speed was reached.