The key is to match the lumen output. A 921 incandescent puts out 320 lumen and draws 1.49 amp. You need to know what chip is used in the fixture and how many there are. Some of the common chips are 5050 at 15 lumen each, 3528 at 5 lumen each, and 1210 at 7 or 8 lumen each. For an example, if a LED lamp fixture has 20 5050 chips, it puts out about 300 lumen which makes it pretty darned close to the output of a 921.
Many of my ceiling type fixtures got the stick-on panel type led fixtures. They come with adapters for several different lamp bases on short leads. They match the 921's very well in color, and the spread out panel actually lights up areas better. I generally use the warm white versions.
