One cross over they never tried----
Fonzie and Richie (mork keeps them looking the same somehow) get transported to 1970's Boulder Colorado bc Mork is lonely to see the friends he made the first time he visited Earth.
The Fonz is amazed how fast both cars--and women go in the 70's.
Both the Fonz and richie also are curious about the equipment called computers--which no longer take up a whole room. And the TV being in color. It had only been black and white in their era. VCRS (remember those!!) did not even exist. Things called microwaves cook food in a matter of minutes.
After figuring out where people go to 'hook up' we see the Fonz try 70's nightlife. Henry Winkler was actually a very good dancer (remember w Mrs. C) Am impressed he had his actual hair pinned up that many seasons of Happy Days while running around in scenes. Takes commitment and stamina. No idea if Winkler himself liked disco....but this show did do what was then current music several times and far more realistically than the later music for Happy Days.
Orson wants to personally meet the one known as the Fonz.... Orson has heard about this being and is in awe. Orson is genuinely in awe of very few if any beings---other than himself. He had tolerated Mindy but this is not actual awe.
The fonz himself is flattered he got to see both earth 1970's culture and the inside of Mork's head. It is a memorable experience for all.
Not sure how they would have filmed this. TV movie?
Maybe it would have been better than the fourth season of Mork and Mindy which had existed. Might have helped get a 5th season out of it.
Mork has to erase their memory once they come back from the 70's though--so they aren't taken away/they will think it's a dream.