"If that be sin I'll make it my morn-prayer."
As Shakespeare has used 'morn' elsewhere but once in a compound, it were better, as the metre requires, to read 'morning', as Hanmer also read.
"Or seem so, crafty; and that is not good."
Editors read 'craftily'; we might also read 'seeming.' "Or seeming so in skill" (Winter's Tale, ii. 1); but no change is necessary. The folio has 'that's'; hence the reading of the editors, the Cambridge included.
"Proclaim an enshield beauty."
The word 'enshield' occurs nowhere else. It is of course, as Steevens says, enshielded, covered with a shield. To this there is no very great objection; but as elsewhere (Cor. iv. 6) the poet has 'inshell'd,' i.e. covered with a shell, it might be better to read so here also, as I find Tyrwhitt has done.
"Than beauty could display'd.—But mark me now."