1.3 Running Like A 1 Litre?

I've just bought a 1985 (mk2) Polo Breadvan, it would have originally been a 1.0, but the guy before the previous owner blew the engine and had a garage put a 1.1 in it, according to the guy I bought it off of, who never had it on the road.

I took it for an MoT the other day, and had the mechanic check the block code, which turns out to be a 1.3.

So I now know the block is a 1.3, but how do I know if the cylinder head is for a 1.3, a 1.1, or a 1.0?

It is running very underpowered for a 1.3, so I'm working on the assumption that the head is smaller. But how do I find out?

Cheers, Ed.M