My old 99 takes a B15 socket and the cylinder shaped (corn cob LED) bulb work fine. They are sort of encapsuled in solid clear silicone rubber. Make sure it's the narrow type, where the end socket is about the same width as the upper part.
Later 99s often have a round more cone shaped lamp, they can take either threaded or bayonet type bulb sockets (usually E14, but I think some sewing machines take golf ball size E27)
For the narrow slender Singer lamp with a left-righ on-off swich and no glass lens, you can max out on wattage. The bulb cylinder is the same width on them, but longer on high wattage bulbs (more of those square yellow patches). If it takes a round golf ball type bulb, there are different options; some have narrow LED fillaments, others have those square yellow patches.
I have had slight problem with some B15 LED bulbs, I had to grind down the two protruding bits on each side, not much, but they didn't turn in place like the old type bulb did. Some are fine though.
This is just a random ebay add, but it gives a good illustration of the narrow cylinder type bulb.