The standard Singer motor isn't very strong at all, but on a 99 it's quite capable with the average flat felled seams on denim. I have stitched through bonded nylon webbing and it does it with ease. Sail canvas just isn't within this models range at all, but I doubt you would get more than two layers under the presser foot if at all. I have the same motor on my 201, and it handles heavier fabrics much better than many fancier machines with stronger motor.
I doubt larger needle helps much if it has to do with punch power, a thin needle tends to bend and break more easily. Go for sharp point needle, size 120; that's within the normal range. Larger can be an advantage if you can find it. I don't know how much cleaning and oiling you have done lately, but a smooth running mechanism makes a big difference on the punch power of domestic straight stitchers; start with needle down, medium speed and it should do the job. It's usuall very densely woven fabrics that need a bit of extra attention to needle size and shape, as well as corduroy, velvets, some type of interfacing, linings etc. Heavier denim and canvas are often easy to stitch through.