Yes, Duratec, Zetec, Ka engine, ST170, any of the Ford inline 4 petrol engines from the 1960's 105E Anglia up to present use the same bellhousing pattern.
You can put whatever engine in you want, thats the whole point. Just the old sierras where a easy swap.
Saw people put 2litre turbos in them, motorbike engines. All sorts.
I've always thought the Mazda RX engine would be good.
Without wishing to be negative you might want to do some research on the fiberglass mouldings and rear suspension geometry on the Avon. The rear tub has the left and right wheel arches in different positions on a lot of them, people have had to chop them around considerably to make them fit. They may (hopefully!) have fixed this in later mouldings (ask on Locostbuilders).
When the Avon came out the IRS geometry was widely regarded as an example of "how not to do it", again this may have been fixed so ask about it.
It's not an Avon, that's what I was going to go for before seeing this one! This is a Stuart Taylor chassis, which is distributed by this place: http://www.ariesmotorsport.co.uk