If you replace the engine in a car, do you have to replace the clutch/sensors/spark plugs(and leads)/thermostat ect? I've been told that you don't have to and you can reuse the parts, but my thinking is if you are replacing the engine, you should use new parts like the ones I've listed and others. Can anyone clear this up for me?

You can reuse what ever parts you like, provided of course they are in reasonable condition. If you replace everything with new parts it will of course cost more than the value of your car.

I'd replace anything that was relatively cheap or that could cause issues if it fails, such as hard to reach hoses and seals.

Components such as the clutch, well, it will be cheaper to do it wjile everything is apart than in the future.

If it's a keeper, replace at will ! 

If the replacement engine has recent service history then there's no need to replace service items.  They're easy enough to replace at a later date.

Clutch & cambelt depends how old they are.  If recently replaced, they can be reused.  Both are much easier to replace while the engine is out.

Sensors shouldn't need replacing unless they were broken before the engine swap.

Thermostat depends on which engine it is.  On the Zetec engine in the Fiesta's it's a pain to access in the car so that might be worth changing if there's no history of it being changed on the replacement engine.  But for other cars where it's easily accessible then not worth changing.

Too many variables to give a simple answer on this.

When I was an apprentice mechanic, my mentor always told me:

"Unless you know you put a clutch in last week, if you ever have to take the engine or gearbox out, for any reason, stick a new clutch in"