Generate data with missing values and specific mean / standard deviation

I generated a variable foo:

Code:

clear
set obs 100
gen foo=rnormal(5,2)

I then do the following:

Code:

gen foo2 = runiform()
replace foo if foo2 < 0.2

Is this correct? Will the mean and standard deviation of foo always be preserved?

Also, how can I randomly replace with . only a certain number of observations?