Going to look at a 1981 Viking 46’ Sportfish with DD8V-71TI

Not in a million years.
18-20 knts max cruise.

Keep token, it may then feel like 24 kts..

You are WAY over-propped. You need to see 350 more rpms at WOT. Is the bottom clean? If yes, it sounds like you either have a serious HP issue, OR are seriously over-propped, which will lead to a much shorter engine life. BUT, you also need to get the trim tabs working and do another seatrial.

Fix injectors first. Boat levelers can usually be changed in the water by a diver since they have external hoses.

Be sure to have all the injectors popped and matched before installing. Just picking up off the shelf is open for to many problems, regardless of the vendor.
My past rants on this has always been rebuild what you have, at a shop that knows their stuff, that does pop and match.
Again, tested, popped and matched before any injector goes in.
Since your going to be down for a few days for a proper injector swap and tune up, take your time and do this right.
I'm hoping you know, you don't just throw injectors in; height timing and rack & control tube travel requires a fine touch. Adjusting all the exhaust valves cold should be done at this time also.
Don't forget new fuel pipes and all torqued to proper specs using the correct DD tools.

If you know all this stuff, great.
It is amazing how many folks don't.
I have been there to fix Buba's work a few times.

Use the serial number to get the correct injector also.
Hatteras and Viking used to order some exotic engine tunes from DD marine vendors. Some strange injector sizes were used. Often, replaced with N90s in a rush or thinking to save on fuel.
Again, Buba in the engine room.

I forget, I had boat levelers many years ago. But was easy to change in the water on my boat at the time....., connect the hose first before trying to connect the ram to the tab......

I remember a tab job that went to pot. The hoses dissolved in my hands. 5200 was used to seal the hoses in the transom.
Look them over well before starting the path of replacement while in the water.
Out of the water may also reduce (eliminate) the ATF spill on the water also.

I believe you have the Nylon ones and they're just driven in, give them a call and verify, but like this

CYLINDER PINS - Insta-Trim Boat Levelers (insta-trim.com)

Looks like your engine has the small injectors......or they were derated at some point in time.....most of the 8v71 TI's I've seen have the N110 injectors.......