What is it?

Some going but not too much stopping. I like the front mudguard.....siamese pipes on a Jubilee/Navigator?

This was a pupular mod to get more power from a Jubilee - drop a Navi engine into the frame.

The frames for both are identical** - with one difference. As the engine is taller, the head steady mounting bracket is higher (& more substantial) on a Navigator. Luckily, the whole front downtube can be removed & replaced by the Navigator item. Or new brackets could be welded onto the existing downtube, or an elongated head steady could be made.

The rear wheel sprocket would also need changing - 55T Jubilee, 52T Navigator

You read about the steering angle being different between the two models, but that is simply down to the wheel sizes & fork differences - the head lug on the down tube appears to be the same angle for both.

As Al wrote above - this would increase the 'go', but the Jubilee front stopper was marginal at best, so you would need to improve the 'stopping' part. Relatively painless approach (but expensive) would be to graft the front end of a Dominator on. A better solution may be to graft on a front end from an old Japanese bike (Some Yamahas of the period had twin leading shoe brakes, even!) - if you can find one (oh Boris, please bring back AutoJumles after Lockdown!!) and know someone handy to make up spacers, etc.

**The frames up to 1963 (both standard & D/Lux) have the same engine mountings for both Jubilee & Navi - up to engine/frame No 106837. After 1963 (engine/frame 106838-on) the rear lower gearbox mount position was moved - as the engine was redesigned, so they are incompatible - so mixing 1961 & 1962 components is easy.