You can also get good motor mount stock from Bill Sawyer.
However, Yellow (Flame) Birch is every bit as hard (and heavy) as Sugar Maple. It makes excellent motor mounts. White Birch is a no-no, as it tends to be rather soft. It is tough, though, so if that's all you can get, use large pads under the engine and you should be OK. Heck, Tom Morris uses Spruce!
White Oak (Pin Oak, for example) is OK, too, but be sure all holes line up and are a bit oversize. White Oak tends to split it stressed by misfit bolt holes. Pecan is good, too. Hickory will work if you can find straight stock.