There would have been engines for the Me 210. Production of the Bf 110 would have been cancelled (it actually was, theoretically, on at least two occasions). Ju 87 production would also have been finished, though where those engines would go is anybody's guess.
The Me 410 was eventually a better aircraft than the much maligned Bf 110. First hand accounts from experienced pilots who did fly the Me 210 tell us that they were very disappointed to go into the Russian campaign having reverted to the Bf 110, but it was one of the best aircraft produced by anybody during the entire war.
The real crux of the issue is why the RLM thought it was getting an upgraded Bf 110, thereby avoiding lengthy development issues, and Messerschmitt sold them a completely new aeroplane with all those issues and some. It is a murky area and surviving records don't explain what exactly happened but there was a level of mutual self delusion at Messerschmitt and the RLM.
Cheers
Steve