Also, minus the bit of reverse taper above the base, it looks pretty decent. The nebari looks really nice, so that's a great start. You could always try to use a sacrifice branch, but I'm not sure how quickly Amur maples grow.
You probably won't be able to correct the inverse taper by scarring, unless you take a BIG bite out of the trunk with a wood gouge or concave cutter. However, you'd also have some risk of dieback at the pruning site, as amur tend to do that, sometimes dramatically, at large wound sites, only to regrow over them agressively.
A better way might be to inarch graft or thread graft a longer limb above into the "necked" section of the lower trunk.
Scars on Amurs tend to roll-over quickly but protrude a lot, at least that's what I've noticed on mine. As Rockm said, you'd have to take a drastic big bite. I've taken advantage of the scarring along the trunk by growing out thick sacrificial branches and pruning them back to the trunk to thicken certain areas by means of the scarring that then occurs. Unless you really hacked back the top of the tree to encourage low branching and kept your fingers crossed for something to pop from below that reverse taper, I'd take Rockm's suggestion.