Do you mean something like this?
f(x) = x^2
y = 5
f(eval(parse("y"))) # = 25this converts the string to a symbol, which is then evaluated (giving the 5 previously assigned to it). Of course the standard disclaimer applies; you should probably put all those "X1045" variables in an Array and loop over the appropriate vectors/higher-dimensional slices of this array rather than looping over variable names.
