Any idea where to find Micro Seiki tonearm parts?

The counterweights or their replacements are occasionally available in Japan. I'd have thought a 'NOS original armpipe screw' would be impossible to find, but could probably be replaced with another '3rd party' screw. I will take a look around for the C/W.

Elizabeth, the other way to do it is to only buy TTs where the parts can be found. For Micro Seiki 1500-series TTs, and things like Garrard 301s, all parts can be found given a bit of time (and the willingness to part with money for them).

Donshoemaker,
For the weights, there is a guy here who sells MA-505-series main weights and sub weights (his store is a metal workshop). The main weight is JPY 9,000 (about US$90), and the subweight JPY 1,000 less (about $10 less). To me that is a fair bit (shipping would add a bit but not much) but 'parts arms' go for more, and I don't know what a machinist where you are would charge. There is another guy who does MS parts and I will try to get his name/site and post it here soon.

Hiho,
No clue other than to wait for one to come around. I know they made them
for after-sale because I saw a table in Japan years ago which came with
something like 5 extra headshells still in their boxes. I think the KP-07M
made by Micro is the only other 'Kenwood family' table with a straight arm
and SME-type headshell mount (and I am only going off what I t-h-i-n-k the
KP-07M headshell is having seen the arm a few times.

That said, you might try something else... there was a cheapo Denon table
made until 4-5yrs ago called the DP-300F. It had the same kind of headshell
I am pretty sure. Given there is a Japanese standard which defines the
headshell collar to stylus tip distance at exactly a certain distance, it will
probably work if you can get the offset angle right. Try googling "PCL-
310" or "Denon DP-300F headshell". The mounting distance
is a full inch shorter (Denon shorter) so the offset angle will probably be off,
but the Denon headshell has long slots to mount the cart so I bet you could
angle the cart out a couple of degrees to get it right. You could probably find
one cheap enough on fleapay to have a crack at it.

Harmon Kardon also made straight arms with removable headshells back in
the day, and perhaps others did too.

The other solution would be to put a different arm on, or just wait for the
Kenwood headshell (either for the KD-770D or the KP-07M) to come up on
fleapay.