I now have my DT400 - arrived here in the Uk fresh from Ohio. Geuine 1 owner bike according to the title and has only 1750 miles on the clock. I have no reason to doubt this as the previous owner was a collector, and judging by the general condition is certainly believable.
In the process of re commisioning the bike, which starts and runs.

First question - are the casings magnesium? I notice on the gearbox sprocket cover  it has "Magnesium" cast into it.
Second question - I read someplace that it is a good idea to draw the flywheel off and check for corrosion behind the flywheel and around the stator coils - is that a good idea?

Picture of the bike attached:


 

Congratulations on the bike and she looks great, I removed my flywheel etc when I cleaned and set the points. It made the world of difference to the bike. Good luck and enjoy her. 

Nice bike, me personally If it starts and drives ok I wouldn’t pull the flywheel, wouldn’t hurt to remove the cover and give it a inspection and cleaning, it is common for water to get in there either through the gasket or grommet that the stator wires pass through. Also a FYI your rear shocks are on backwards the oil reservoir should face forward.

Reason for removing flywheel and cleaning is corrosion debris can allegedly ruin fragile CDI pick up coil.

Side covers are magnesium.

I highly recommend removing ignition and oil pump covers after washing or operation in wet conditions to make sure things dry out.  A few drops of water will wreak havoc and it's very common. 

Welcome from NZ. Probably more important to pull the wheels & check the brake linings. But looking at the bike condition i'd say you won't have any worries there. I'd be keen to get on & ride if she's as good as it looks. If its been sitting for an extended time it'll likely smoke the village out so everyone will know who did it lol. Oil is possible to gravity into the motor if the miniature ball & spring has any bum fluff around it & stuff seems to get there in with the oil over 45 years. Don't worry about that just yet but start it & run it up slowly increasing revs so as Not to throw excess oil up to the plug in one big rev. She'll be good.

sorry ignore most of my woffle above. I see it runs. I'm half asleep.

“Reason for removing flywheel and cleaning is corrosion debris can allegedly ruin fragile CDI pick up coil.”

Agreed if after inspection it has corrosion and is dirty, hence my statement of removing cover for an “inspection” I have had a dozen or so of this model of bike and it seems about 60% of the time yes they do need the flywheel to be removed and cleaned, the other 40% of the time they have been spotless and keeping with my line of thinking if it isn’t broke and runs and drives as the OP states I would leave it alone, I have seen numerous flywheels and coils damaged by someone that did not have the proper tools or knowledge on how to remove a flywheel, and if they did get the flywheel off without damaging anything they failed to properly tighten the flywheel causing more $$ damage when the flywheel came loose and tore up the complete pulsar and stator assembly and crank.

Thanks for everyones advice - I will at least take the flywheel casng cover off to take a look inside, although it is currently stuck in place - any recommendations for getting the cover off, guesing gently tappling with a rubber mallet and perhaps applying WD40 or similar to the gasket joint, main thing is it not to hurry.

If everything looks good under the cover I will probably leave the flywheel well alone, as I am great believer in if it aint broke dont fix it.

Oil pump cover came off OK, there was a bit of moisture in there but nothing to worry about or cause any damage, the bike has been well looked after by the looks of it.

It starts pretty easily so cant be all bad,

Over time "a little bit of moisture" will make it look like everything under the ignition or oil pump cover was stored on the bottom of the ocean for 20 years. 

It's just prudent to check and let it dry out. 

Agree !!