Shame on you for even considering purchase of an OMC.
It sounds like you are getting pretty close to having the 1000 dialed in.
I'm thinking that the noise you are hearing as you shift it into Forward Gear is simply caused by moving the shifter forward too slowly. Find the range of the control handle's movement from Neutral to fully engaged in Forward gear, but before the throttle control cable starts to move by having a pal watch the port side movement of the shift & throttle control cables at the engine. Then you will know just how far you need to move the control handle to engage the Forward shift. Once identified, you will want to push the control handle that far promptly rather than slowly.
Note: It is a bad idea to attempt to shift the controller into Reverse unless the engine is running, - or - with the engine turned off have someone rotate the propeller as you shift to Reverse.
Then the stall as you try to throttle up is most likely caused by having the idle jets set too lean.
The Merc does not have accelerator pumps in the carburetors, so it is dependent on some extra fuel slobber laying in the throats & crankcase so that when the butterflies are opened there is some fuel mix already present to mix with the big blast of air coming in.
When looking at the fronts of the carburetors from inside the boat, the idle jets are the recessed screws seen at about 1 o'clock outside the throat of the carbs.
They are probably currently set somewhere between 3/4 to 1 turns counter-clockwise from being fully closed.
You may have a very crisp & sweet idle going on at this setting, but once you get all three carbs set the same & in the aforementioned range, you will need to open all three needles an 1/8 of a turn c-clockwise & try to accelerate again.
If it still stalls, repeat the motions by another 1/8 of a turn.
Repeat this until the beast accelerats for you without hesitation.
You might even end up with the idle jets set as far open from fully closed as much as 1 7/8 to 2 turns, but I doubt it will need that much on the old TIGER 1000.
And,...Next time you need a part, please post here. Lots of us have some pretty decent piles of used, but good parts.
I doubt that I would have charged you more than about $5 + postage for one of those $130 float chamber tops.
Please post with results after completing the tests suggested.
Doc Frankenmerc