I am not a marine
architect, but do have an Islander 37MS. This is just my uneducated opinion.
I am understanding that you are considering moving the mast
, boom, sails
, stays and such from the Roberts to the Islander.
I don't think your new Islander "J" will =17.17
The islander has a keel
and the new mast would need to go in the same location as the old and the fore stay will attach to the same location on the islander's bow as before, so the "J would still =14.5. You would gain a little sail area on the jib
from the extra height, but not much. I would not expect the Roberts Jib
sails to fit as the fore stay of the Islander with the new rig will be shorter then it was on the Roberts, and the clew would be rather high off the deck
The "E" that you show of 15.5 is for the standard I37 not the MS.
The "E" for the I37MS is 13.5 so you should not have any issue on the main, as it should remain about the same sail area.
The "I" will only =44.83 if the mast of the roberts is about 2' longer then the islander as the Robers and The islander have different mast step to deck
measurements, and "I" is from the deck up.
I would also check the mast to chain plate
angle for the shrouds to make sure they match up and that the spreader width for the two rigs are the same.
So in general you should end up with almost the same rig as the original, with the exception of about 2' extra mast height. (if the mast is about 2' longer). The Roberts main sail will work and the islander Jibs and Genoas would work with a pendant.
The I37MS is a great boat, but it can be a bit tender
and pick up a bit of weather helm
with too much sail up, So I might wonder if a higher mast may make it a bit more tender
My I37MS balances well and goes like a bat out a hell to weather, or on a reach with the standard rig, but needs a BIG sail out front to go downwind fast.
I should note that I have rebuilt my rig from the mast step up, and would not change the sail plan much. The original rig was a bit of a put together, and the boat would go fine but become rather hard to handle at all points of sail at 6-7 kts
Best of luck