Could you transplant the engines into an older model that had more lenient emission regs?
Depends on where you live. For example, in CA, if the car was required to be checked (certain model year and later), you would have to meet the emissions standards of the year of the newer engine. If the car isn't required to be smogged, then no worries. If you had a 1980 model, and swapped a 2015 engine in it, then your 1980 model would have to meet 2015 standards.
“Could you transplant the engines into an older model that had more lenient emission regs?”
Even easier, buy a beat-up same-year gas car and put the diesel in that. But no promises regarding parts, if the engines are actually disposed of.
I kinda want to stuff a TDI into an early model Porsche Boxster picked up cheap after IMS bearing failure.