Although I didn't build a crib I remember doing this a few years ago and searched for plans in vain because the manufacturer of the crib had gone bankrupt. In the end, I winged it. (I didn't build the crib but converted it into a double bed.)
In fact, it was fairly straightforward. The long sides of the crib become the head and footboard, and you add stretchers. Because you know what the finished bed dimension must be according to your mattress size, and crib mattresses are also a standard size, you can work it back.
I would go down to a baby store and photograph and measure the crib you like and copy that. The most important thing is not to make the spaces between the bars too wide. I think there is a legal minimum.
