I've made these mockups as a reference, based off of Link from Zelda II. Far left is the original SMBX's Link as a base, to the right is SMW's Mario as a reference.
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From left to right the changes are:
1st SMW Link: Mostly a recolor, eyes, hair, shading and ear have been redone.
2nd: Changed the face perspective to better match SMW's style.
3rd: Bigger head (and nose), shorter body, hat rim.
4th: More curve for the hat, cheek detail, shortest body, sword and shield more accurate to SMW's style.
5th: Fuller hat tip, wider head for longer sideburn, shorter arm in favor of a curvier shoulder, longer legs, no boots rim.
6th: Ear farther apart, wider neck, Luigi's skin color.
Choose whichever or tell me which features you'd like combined or changed.
About the tunic colors:
They are in pairs, to the left is Mario's skin tone, to the right is Luigi's skin tone.
Top left: Luigi's green.
Top right: SMW's most common green.
Bottom left: Fire luigi's overalls.
Bottom right: Bowser's green.