Like Redsquirrel said, if it's a spoked flywheel, then you are good to go -there is only one size, but it's larger than the solid one. The solid style is designed for use with a motor. A motor can be mounted and used with a spoked flywheel, and often the treadle machines were converted to electricity at some point in their lives.
Since you had said you converted it to Handcrank - I should have known you already had the correct wheel LOL!! Sometimes my reading skills aren't what they should be.
Now you just need to get the base. BTW belts are easily obtainable - usually for $5-$10 range. I don't know how familiar you are with treadles - the normal belt is one long piece with a staple, and you cut it to fit and then use the staple to join the ends together.
The neat thing about most of these old machines with rectangular bases - they are almost all interchangeable in the cabinets. So you don't need a Singer base, especially if you are going to make your own top.
If you are looking at Singer bases, make sure they have all the parts. The two most likely missing are the belt guard and belt shifter (some really old bases may not have that). I like the belt shifter - if you have the cabinet style that you need to tilt the machine back to open the brace and then lower the machine - you need to take the belt off the treadle wheel first. Other style cabinets (mostly non-singer) where the machine pulls up with the cabinet opens don't need this and the bases don't have the belt shifter on them.