I've seen that conversion done a few times and while it can work, the end resulting trailer is typically much heavier than a purpose built utility trailer and really decreases the total capacity of the trailer.
For instance, if the boat trailer was rated for a weight capacity of 2500#, adding decking and side rails more than outweighs what removing the bow winch stand, and rollers and/or bunks give back, so you end up with a trailer that's only able to hold 1800# safely. And remember, unlike a purpose built utility trailer, the substructure of the trailer isn't around the whole perimeter but the front half of your decking will be unsupported by the trailer's frame, unless you're going to weld some additional metal to the trailer, which opens a whole different can of worms.
What I'd have done....sell the boat trailer as it was, even the worst seem to bring $400 and up, then go to Lowe's or Home Depot and buy a purpose built utility trailer. One of those in a 5' x 8' configuration runs around $700-$800. I'm guessing between the extra materials you'll need to convert the boat trailer to a utility trailer, not to mention the time and effort you'll expend, will about make up the difference that you'd have to spend between what you could get selling your old trailer and buying a new one.
Just a thought............