"Pin number I've used is 91608-20010, it wasn't a GT80 pump though and I don't know how many different pins they might have used.

Make sure you don't press pin in too far or it will jam the pump plunger.

Here are specs for 1978 GT80E and 1978 GTMXE, same engine, should be close."

Thanks Mark!  Do you happen to have one of those pins handy to measure for me?
I may wind up going down the road to Grainger.

I made a note to myself not to go in too far with the pin.  Any suggestions on getting it in?

That '78 info on output is perfect for me.

I've read a little about color coded oil pumps.  Mine has what I'd call pink paint applied as a thread seal but the closest I've seen in docs is red.  Any idea what the pink indicates?  Or is this one simply red?  

I do not have a pin. 

Looking at the part number, the "9" series sometimes has dimensions coded into them.  My best guess is 2.00 mm by 10 mm long.

(Note that while Yamaha is "metric", some parts and threads are "standard" so don't assume it must be metric)

I wish to correct my earlier post on the pin sizing . The dimensions I gave you were for another pin in the pump .
The part number Mark has supplied is the correct pin , but no dimensions were provided with that number in the 
parts book . NOS units are available on Ebay , relatively cheaply .
The correct sizing is important . If the pin falls out , the oil pump will only provide Idle quantities of oil , so you could
seize your bike . I know of an example of this . I'd use the OEM pin as cheap insurance against this happening .

Thanks Mark! Thanks Swoop!
I appreciate your replies and the info you've provided.

I have a Grainger close by and can buy a bag of 50 or 100 - 2mm x 10mm roll pins for the cost of one or two on ebay. I think I'll know when putting it in, how likely it is to fall out.

I'll let you know how it works out.
ZT

2.5 roll pins on Amazon of varying lengths. Another option is this link & pic if it'll load & would need a threading tap probably 4mm but might be 3mm. Other thing 'is' if you end up replacing the roll pin be very careful not to insert it so far that it hits the shaft & stops it going out & in.
www.amazon.com/Spring-Tension-Sellock-St...4/dp/B07ZD2B5F2?th=1
Original oil pump disc kit oil pump pulley Yamaha DT 125 R
eBay item number:184890901969
My pic of the ebay item won't load. Hope the number works.

Here's a link showing the threaded guide pin in place
www.cmsnl.com/yamaha-rt180-1996-t-usa_mo...ump-assy_5251310101/

Thank for this additional info, RT.
I have one of the NOS 91608-20010 pins on the way and hope it works out.
ZT