A few general points:
To me, "make oneself fit", though not actually wrong I suppose, is less natural than "keep fit" (if you're already fit) or "get fit" (if you're not).
"Keep one's body fit", though again not wrong, is also less natural to me than just "keep (oneself) fit" (unless I particularly wanted to emphasise physical fitness over, say, mental fitness).
I would personally more naturally say "they/the girls exercise" or "they/the girls do exercises", rather than "they/the girls do exercise". But "do exercise" may come more naturally to some people.
"Some exercise" suggests that you are either describing a particular exercise session, or perhaps saying that they do a little exercise but not a great deal.
Otherwise:
(a1) The girls do exercise to keep fit. -- OK.
(a2) The girls do exercise because they want to keep fit. -- OK
(b1) The girls do some exercise to make it themselves fit.
(b2) The girls do some exercise to make their body bodies fit and healthy.
(c) Do Doing more exercise will keep our body bodies fit.
(d) They exercise every day to keep their body bodies fit.
(e) They are doing exercise because it can make them as fit as a fiddle. -- To me, not very natural. "because it can make them" doesn't read too well, and "fit as a fiddle", though a well known expression, doesn't seem very natural in this sentence either.
(f) They exercise every day so they will be fit. -- OK