First I would take it apart and gently rinse the reed plates off with slow running lukewarm water. Then pat dry (don't rub, you could snag and bend a reed) with paper towel. Dried saliva or other stuff on the reeds can change the pitch. See if it is tuned any better.
Next look at the gaps and see if they match. If you see a reed that is gapped very differently than it's neighbors then gap it to match.
Warm up the harp for 10 minutes by putting it in your pocket.
Next see if the top row is in tune with itself. Block the bottom row with your lip and play some scales. If you are happy with the sound of some tunes played only on the top row then you can go striaght to adjusting the tremolo rates. You can also check the tuning with an electronic chromatic tuner. I like the online tuner at seventh string. https://www.seventhstring.com/tuner/tuner.html
That will not work in Chrome but it will work in Explorer if you have Java.
The reeds will probably be a little high because harps are usually tuned to 441 or 442. Adjust the tuner so that most reeds are in tune and see if any of the top row reeds are very out of tune. Because of the nature of harp reeds with tuner needle will not be steady. That is normal. If a reed is obviously out of tune then tune it. Scrape a little near the tip to raise the pitch and a little near the base to lower.
Next let the entire harp dry out and go back to room temperature before adjusting the tremolo rates. If you have been working with the top reed they will be warmer than the bottom row and may also have some condensation on them.
OK. Warm up the harp again.
Next check the tremolo rates by playing the top and bottom reeds together. If the tremolo rate (wavering, beating, etc.) of a note is much slower than it's neighboring notes (or not there at all) then speed it up. (If there is no tremolo make sure the bottom reed works. You may be hearing only the top reed. Block off the top row and play the bottom row to make sure both reeds are sounding.) To speed up the tremolo you scratch a little near the tip of the bottom reed (which should be pitched a bit higher than the top reed, check with the tuner if you think it isn't). If the tremolo rate is much faster than the tremolo rate of the neighboring notes then slow down the bottom reed by scraping a little near the rivet end of the reed. These are very tiny adjustments. Go slow. You can always remove more metal if your adjustment didn't change anything. Plink the reeds after adjustments. Be careful not to mar the edge of the reed and create a burr (but those are easy to knock off if you do, a burr can hit the sides of the reed slot and cause trouble). If you think the top reed is a bit flat and the tremolo rate is too fast you can also speed up the top reed to lower the tremolo rate.
On a normal tremolo harp the bottom row is slightly higher pitch than the top. On such a harp speeding up the top reed (make it nearer the pitch of the bottom) will slow the tremolo. Slowing the top will speed up the tremolo. Speeding up the bottom reed (making it even sharper than the top) will speed up the tremolo. Slowing down the bottom reed will slow down the tremolo.
I would say go for it. If it not usable already then what do you have to loose? Or send it to a pro. Suzuki USA in San Diego has repair folks. I don't know the cost.
It is normal for brass reeds to settle and drift a bit with age and metal fatigue. I have tuned all four of my Suzuki tremolo harps just a little bit on just a few reeds and they have stayed in tune for years afterward. If a reed is very, very far off it probably has a stress crack. It will not stay tuned to whatever pitch you make it and will eventually break. Check out Pat Missins thoughts: http://www.patmissin.com/tunings/tun10.html
Here is a good tuning tutor (and these tools can be bought from Rockin Rons Music):