I've not met a Yanmar
I guess it's a two cylinder engine
cooled, with vertical cylinders. Is that correct?
The engine should have one or two information plates, perhaps telling you where it was made and when (although sometimes only in code in the form of particular alphanumeric characters).
I think the T series started production in the mid-1970s.
T for Nihongo tatesui: sui, being a Chinese reading of the kanji meaning water
(and more usually pronounced mizu); and tate for a kanji meaning upright or vertical (in contrast to horizontal cylinders).
But I suspect the T prefix may also have been used earlier for small engines not intended to be exported from Nippon.
How old is the engine? Marinised or not? Made for industrial, generator
, or agricultural use.
Many (?most) T series engines were made for industrial use, some for agricultural use (small tractors and appliances
used in rice farming).
, the TM family
, were marine
engines (and books
.google.com has much of a manual for the TM family).
The Yanmar plant in Europe
made many T series engines, including some of the TN family.
A Yanmar dealer, perhaps a Yanmar agricultural or Yanmar industrial dealer rather than a Yanmar marine
dealer, should be able to help.
search quickly turns up retailers claiming to sell manuals
for T, TF, TN, etc engine families.