The bolts themselves, or the mounting locations on the cases?
A number of years used the same basic engine, and varied how the engine to frame mounts were configured, to fit the different chassis setups. The bolts themselves won't have changed too much. They need to be the same size and same length to pass through the engine correctly, and hold it in place.
I would not expect every YZ made in the last 30 years to use the exact same length and diameter bolt.
That said, its not rocket science, its a bolt. It has a diameter, a thread, a length, and a hardness.
The diameter wants to be right. Too big won't fit through the hole and too small will not be strong enough and will allow things to move around.
The thread matters most when it has to thread into a fixed part. If it is a through bolt with a nut on the other side then fine/course doesn't matter much.
Length can have a lot of range to it. You want the bolt to extend all the way through the nut, showing at least one thread beyond. Anything more than that is unnecessary, although often extra length is handy in getting things drawn together. If the bolt becomes too long then the excess can interfere or you can run out of threaded portion of the bolt and can't tighten it down properly
The hardness is perhaps the most critical and most often overlooked parameter of a bolt. There are three common grades of bolts: Grade 8 is very hard and very strong. Grade 5 is reasonably hard and strong. Ungraded bolts are cheap, soft and weak, don't use them on any critical function.
If you have a bolt off an older engine mount then it is probably of a suitable hardness. If it otherwise fits, go ahead and use it.
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