I need to qualify this, I am a 2 stroke guy and I haven't tested what I am suggesting below on a GPRXP.

But if I was building a 4 stroke GPR conversion ski I would seriously consider using parts from the GPR Bullet handling kit.

Specifically the transom, pump shoe, intake grate and front half of the trim tabs.

The shoe and intake grate are machined billet aluminum
1) they won't break
2) they will provide improved hook-up (should help with all the HP you are trying to harness)

The Bullet trim tabs will improve handling.

I would also use a Skat pump.

Bullet / Skat equipped hull for sale for a reasonable price