How much can you trust a reference of a paper that is not available
Is there a way to express this (not as a citation format), that maybe this data is accurate, but not sure, since it is the only available source about that data.


How about something like: "X, measured by Jones in 1950, is commonly given as the value for Y (as cited in Smith, 1989 and Cutler, 1995). However, the original manuscript by Jones describing this result is not generally available."


  Or should I ignore it all together and pretend as if there is no data.


Don't do this. If X is cited often in the field, then by pretending as if there is no data, you give the appearance of not knowing your field. 

Whether or not you should use X (knowing that it is unreliable) depends on your purposes.

If your work hinges on having an accurate value for X, then you definitely should not use X if you cannot track down a reliable source for it. (Nor should you make something up.)

If you are using X just as a "sane" value for something, and it isn't central to your work, on the other hand:

If you think you have a better value than X, and can justify why you are using a different value than everyone else in your field, go ahead.

Otherwise, if you need a value, you should just use X (unreliable as it is). A bit of unreliable data that is accepted in your field is still better than a bit of unreliable data that you just made up with no justification.