look this is a similar issue with 4age ke70's.
essentially, yes, it is illegal, (by law p plate laws restrict any engine modifications, or by any means any engine conversions.)
but, and many people can be witness to this, if you keep it neat, tidy and relatively stock looking, aided by a quiet, exhaust not a stupid cannon or whatever, and a clean car, not mega poking wheels or eye catching paint jokes, theres actually a fair probably that no one would look at you any differently.
I guess one thing on your side would be, the 5k doesn't look out of place within the engine bay. They look pretty similar, era specific.
additionally, if you can find someone who's either oblivious to these motors (and wont know any different) or someone willing to overlook the conversion, you can have the engine numbers changed on the rego papers which will only help your defense.
Some people might flame me for telling you this, and yes, its a risk, but will the right attitude theres a good chance you could do this and be untroubled by the authorities.