The Peco 3 piece sports exhaust seems a popular one. The manual suggests a slight bend in the centre section but I managed without.

Despite being for an MGB most of the exhausts may require a little tweaking to fit an NG, this can usually be done cold over suitable lumps of wood and in a vice etc. The most important thing is to allow yourself as much ground clearance as possible without vibrating against the chassis under load.

The exhaust will need tweaking to fit an NG, I personally used a cheapy 3 piece system from MGBhive and had no problems. If you wish I will send you a copy of the of the manual pages that relate to exhaust fitting.
ATB
Mark