Exhausts and exhaust manifold when fitting a larger engine

[vwstanley66]

I have replaced my original 1.7 non turboDiesel engine (1989) with a 1.9 non turbo diesel. I used the exhaust and exhaust manifold from the 1.7 engine.

Is this ok or should i be fitting a 1.9 set up and will doing so increase power?

The van cruises at 60 and can reach 70 ish but feels slower than when fitted with original 1.7 engine.
The vehicle in question is a type 25 campervan.

If I do need to fit a 1.9 set up any idea of good places in the west midlands to buy or fit for me?
Thanks