.. or, if you want to do it without any brain strain -
Recalling that
(a) at the stall, CL = CLmax and is presumed to be constant
(b) L = W, if we make the dubious presumption of level flight
Then, where "1" indicates before, and "2" after ..
CLmax = L1/(1/2.rho.V1^2.S) = L2/(1/2.rho.V2^2.S)
so, cancelling out the common factors
W1/V1^2 = W2/V2^2
from which you get either
V2 = V1.sqrt(W2/W1), or
W2 = W1.(V2/V1)^2
So, with the example figures given, and presuming that the speeds are CAS, not IAS
W2 = 1000.(50/45)^2 = 1234.6 (call it 1235)
This only has some validity for a constant configuration comparison.
Also, and this is very important, PEC gets interesting around the stall so IAS won't hack it (we've all seen a Cessna with naught on the clock at the stall - means nothing due to PEC) so you have to do the sums in CAS to get anything in the way of sense out of the sums.
I think that this was where Matt was coming from ?