“made lame by Fortune's dearest spite”
'Lame' here means physically crippled or disabled — the poet presents himself as diminished in body or circumstance. 'Dearest' does not mean 'most beloved' but 'most severe' or 'most costly' (an archaic intensive). Fortune's 'spite' is her malicious, wilful injury — the harm a fickle goddess inflicts.
classical Fortune and her wheel“beauty, birth, or wealth, or wit, Or any of these all, or all, or more, Entitled in thy parts”
'Wit' here means intellectual quickness or natural intelligence, not jokes. 'Parts' means personal qualities or gifts — what a person is made of. 'Entitled in thy parts' means 'legally invested or lodged in your qualities' — the Youth holds title to these gifts as a landowner holds title to an estate. The accumulating 'or any... or all, or more' is the poet's way of saying he is not limiting his claim: whatever the Youth possesses, all of it or more, the speaker wants to participate.
“I make my love engrafted, to this store”
To 'engraft' is a horticultural term: you cut a shoot from one plant and splice it onto the rootstock of a stronger one so that the cutting draws life from the root and grows as part of it. The poet's love is the cutting; the Youth's 'store' (stock, treasury of gifts) is the root. By grafting his love onto that root he claims a share in its vitality — he is nourished by what the Youth has even though he cannot grow it himself.
“this shadow doth such substance give That I in thy abundance am suffic'd”
'Shadow' and 'substance' form a paired opposition the Elizabethans inherited from Platonic thought: a shadow is a mere image, a reflection with no independent existence, while substance is the real thing. The poet calls himself a shadow — dependent, without solidity — yet argues paradoxically that this shadow 'gives substance': the Youth's reality is so rich that even the poet's reflected share of it is enough to fill him. 'Suffic'd' means satisfied or made content.