... but failing and having to paste in the "Happy Days" characters.

I mean we have this time machine that is seemingly bigger on the inside than the outside. Said time machine constantly malfunctions and can never get the characters to the place they want to go (Milwaukee, 1957).

Just switch the time machine in this show for the TARDIS and the Happy Days characters for the Doctor and Co around the Hartnell/Troughton eras.

The only difference that I could see was that, unlike the TARDIS, the time machine was confined to Earth. It could go into the past and future, but not to other planets.