An excellent project that deserves to succeed. It won't be easy but you'll have a remarkable car when you've finished.
If you can find one then a 1500 or 1600 Kent engine will be excellent. They are very basic engines and anyone with a modicum of mechanical knowledge, and a Haynes Manual, will be able to work on it. You should establish a source of parts before you commit yourself because these are rare engines these days and Ford gave up their "10 year" support regime decades ago. Engines from any Mk1 Cortina will fit and some from the Mk2 too. Check which side the exhaust manifold is on because you may have to prepare for the difference between a cross-flow and "conventional" cylinder head.
A zetec engine is a more complex beast, spares should be available for most models. But be careful about fitting it. Check every dimension before you spend any money. The mountings will be very different from that of an Anglia, and you will also have to make sure that clearances for underframe equipment (eg steering gear, exhaust pipe run, etc) are adequate; consider the runs for accelerator cable and electronics, as well as the fittings for the EMS. You will almost certainly have to take the gearbox along with the engine and this may mean some bodywork changes too, perhaps even a modified prop shaft if you're really unlucky.
If you have to change the gearbox don't forget that the speedo may need replacing or recalibrating because the gearbox speedo drive ratio, and possibly coupling too, will not be the same as the 1965 Anglia one.
Make sure that the suspension is ok too.
If you succeed in fitting the zetec engine remember that even the 1600cc is much more powerful than its Kent equivalent so you can expect a significant insurance hike.
You'll have to tell the DVLA whatever engine you fit because the engine size and number will have to be changed on the registration document.