You have Old Man Willow but need support.
Look for Dunland Crebain or Many Partings.
Expendable material and a real return target. You cannot sacrifice a nontoken artifact such as Lembas to Willow unless it is also a creature. The cost says another creature or a token.
You already have Many Partings.
Consider Old Man Willow if it fills a useful job.
Partings finds a land to hand and makes Food. Willow grows with lands you control; on its attack you may sacrifice that Food to shrink an opposing creature.
The support is missing
Take castable creatures and answers. If the engine never arrives, use the good cards rather than forcing narrow support.