Any Jet Boat experts on the forum ?

[outdoorlad]

First thing to check is that the grill is clear of stones, then shine a torch up the nozzle and check the impeller clearance to the wear ring, the tips (trailing edges) may need building up.

[Barny Rubble]

Looking at your photos it looks like your boat is a J31. These were a Marlborough Marine Dart hull that were usually out board hulls but some were used by Hamiltons as jetboats.
These hulls are deep v and are around 20 degrees so are more suited to deeper water than shallow water.
Because they are a deeper v they will need a higher speed to plane and because they are a short hull as well are sensetive to weigh/load.
The Holden 3.8 should be more than enough power but as it looks like a colorado single stage you are at or over the top end of H.P. for this unit.
I am guessing that the previous power unit was matched to the unit and now that the 3.8 has been installed the extra power combined with a worn unit is causing the problems that you are experiencing.
Idealy a Holden 3.8 would have a 2 stage 7.5 inch hamilton unit behind it to efficiently use the available power but you could get away with a single stage providing you got the correct impellor and nossle to suit and the rest of the unit is in good order.
You could get the bow down by shifting weight forward or pointing the nossle down further but watch out for tendency to bow steer.
Also check the bottom with a strait edge on a fore and aft plane to make sure that previous repairs have not altered the integrity of the planing area.