I need to substitute fresh yams for canned, but I want the sweetness

From Bruce's Yam's:

Bruce’s canned yams (sweet potatoes) use only the tender most part of the sweet potato called the sweetheart. As the number one selling canned sweet potato in the U. S. it has been referred to as America’s favorite – sweetheart. Certified by the American Heart Association.

From McCALL FARMS:

Ingredients: Sweet Potatoes, Water, Corn Syrup, Sugar.

So basically, only use the sweetheart (center) of the sweet potato, and add a good amount of sugar and/or syrup.

If you want to devote the time to get the maximum sweetness, you can take advantage of natural enzymes to accomplish your goal. If you can hold your yams between 135 and 170°F (57 and 77°C) for a couple of hours, you can dramatically increase the sweetness. They will still be hard at this point, so will need roasting to soften. A sous vide device will be the best way to do this, but you can do a decent job of improving sweetness by roasting, wrapped in foil, in a 300 F (149 C) oven for about 2 hours. The science and process is explained in detail here.