New engine has been fitted to Fiat Brava 1.6 but car will only start when towed and then runs well. Can anyone solve the problem

most basic.... starter turns over? is it turning over ok?
is battery well charged?
when towed, does engine start fast(as soon as u relise cluch ) or will it take time... lets say 5+ sec?
if it starts, is it starting smooth right away or will it first work like on 1-3 silinders - not eaven.
enine was from other car? was it running on other car.. did engine had any longer storageing a (had not been started for)year or so?

When starting it by towing does it run ok? How does it behave? Is your battery fully charged and in good condition? Have you tried starting with jump leads?

More information is needed really but based on the small amount of information;

The problem is most likely a faulty crank sensor. At low rpm (ie when starter motor turns the engine) a faulty crank will not produce a good enough signal, but at higher RPM the crank sensor works well enough.

First I would check the crank sensor/phonic wheel clearance (Should be 0.5 to 1.5mm). Also check the wire/connector block for damage/corrosion. Then try another crank sensor.

If the crank sensor is giving a poor signal you will not get a spark or fuel injection, so you could also check that before spending money.