I have a pioneer pl-51 direct drive tuntable I bought back in the early 70's. last time I used it was around 1989. it doesn't have a needle because my grandkids broke it. is it worth buying a needle to use it again?
i have a grado cartridge, last i heard they still have a replacement stylus for it.
I would advice you to find a good Brick and Mortar store and have him set it up for you in your home. Makes a big difference.

Dear @g_nakamoto: Mainly depends on what level of quality performance you want to listen and how many LPs you own.

Pioneer design good, very good and excelent levels of products. Your TT model is not really in any of those levels for today standards, it has really poor measures as specs.
Of course that can works at some limited quality level performance. Its tonearm is not good neither.

You can get an inexpensive Pioneer vintage TT looking at the 600 series, as the PL 630 that's very good motor with good tonearm.

As I said all depends on what you want.

Regards and enjoy the MUSIC NOT DISTORTIONS,
I bought a PL-71 in 1974 from Radio Shack, when I was attending a university in Cookeville Tn.  Back then, there just weren't many Hi-Fi stores around, unless you went to Nashville, even then maybe a couple.  Audio Systems and Nicholsons.  Anyway, I stopped playing vinyl around 1987 and went to the CD.  About 8-9 years ago, I decided to get my old Pioneer back out and started with replacing the stylus in the original shure V-15.  After trying a few different cartridges, I installed a Koetsu Urishi Black.  It sounded awful until I beefed up the mass of the tonearm.  The difference was like day and night-truly.  I lubed the spindle and cleaned the pots that adjust the speed and the performance of the old Pioneer is very good.  Some can't tell the difference between it and my VPI Scoutmaster.  Proper motor speed, good flawless arcing, sufficient tonearm mass and of course proper cart set-up is all I need.  The main thing I found, was the stock tone arm just didn't have enough mass.  You really have to experience this, to hear the difference and it's amazing what increasing the mass will do, for the sound.  Definitely keep the "51", get a good cart and experiment.