take the head off, measure the distance from the top of the jug to the top of the piston at BDC. then measure the same distance but with the piston at TDC. the difference in this measurement is the stroke.
and take the intake off and take a picture of the intake of the jug. we will know if it has been ported.
yes you can. but i wouldn't suggest installing the HG right now. if you are planning on getting work done, depending where, they will have a HG on hand and will need it to correctly cut your head and port your jug (the base gasket, which will need replace if you take the jug off)
who ever does the porting and head cutting will need both the base gasket and head gasket you plan on running. different gaskets offer different thickness, this will change the port timing and squish of the head.