rewiring a 15-91 is not for the inexperienced. the $150.00 seems a little pricey, but it depends on what you get.
for that rice would expect to replace the light assembly, disassemble the motor completely, new wire to the field coils
clean the commutator, degrease the motor housing replace brushes if necessary. remove all possible parts, for cleaning/degreasing. supply new 123 cord, new bobbin case, new electronic foot control, new needle and slide
plates, new bed tension. obviously reassemble and test sew out.
jimthegoatman