The GRP plug is a bit shorter as the Nova plug. So the GRP/Ninja burn chamber is too short to use a Nova plug, it will protrude the burn room.
It doesn't matter that the GRP plug is a bit too short for the Nova burn room.
The only thing it does is creating a bit more volume, so a somewhat higher top speed (2-3 Km/h).
It does create a good seal and that is the thing that really counts.

The GRP plug is far superior to the current Nova ones. With the Nova ones it is always a bit of a gamble if it will last a main final or destroys your engine halfway.
The GRP plug is also available under the Max brand, so you could go for that one as well if you still not trust it.