This is  FS5C71A no brass... and holds 275bhp just fine.

Not too many 71A, B or C:s for sale around here. Maybe before anything else I'll take my three-teeth-missing 71B to a professional to see if it could be fixed. In my non-professional opinion it looks pretty good and feels tight. Worth a try at least.

Do you do the lottery?

https://www.rhdjapan.com/kameari-super-close-gear-set-71b-type-a-s30-s130-gc10-gc110-gc111-r30-c31.html

Er... do you mean that I shouldn't try to fix my old box because the result would be a lottery? Or "try these rhdjapan parts, they might work or not..."?

Better buy a good used..   It is a lottery to repair it, if you never have opened a gearbox, it;s a big risk.. but on the other hand, if you have the parts laying around just try to repair it, you learn a lot.

Now I have one more option to deal with this.

A local Nissan club member not so far away just gave me an offer to buy two broken 71C-boxes for 100euros. One misses teeth from third gear, the other has a broken selector fork (is the part name right?) He told me he believes that I or my "team" could create one good box from them.

I would gladly buy a good used one. Finding one is hard. 

Don't forget that you'll need to have the 71B case machined to fit the 71C oarts, in order to use a 71C on an L6 engine.  Also, the 71C 3rd gear is probably the same as your 71B third gear if it's an early 71C  So, really, the fastest way to get back on the road is to just swap in a used 3rd gear.

There is little strength benefit to the 71C unless it is a late 90's 71C with the wider gears.

How do I know about the gear widths when the box is on the floor and seller knows nothing?

Er... do you mean that I shouldn't try to fix my old box because the result would be a lottery? Or "try these rhdjapan parts, they might work or not..."?

I mean they cost a lot. They would be very nice.