1982 KZ1100-D big bore

I was told my bike is a 1100cc bored to 1390cc. That seems like a pretty big increase. Can this be correct and if so I could use some help finding a head gasket for it. Thank you

It will have a big block fitted which is a drag race item and should be 1395cc to be more accurate.
There's not enough meat on the std block for it to be bored out that big so your block may have Wiseco ,APE, STAR or Orient express etc cast in to the outer faces as these are some of the main producers of these blocks.
Wiseco , Cometic , APE or Pitstop Performance etc should be able to supply a head gasket but it will most likely be a copper one.