Syntax for replacing missing values with the Mean of Participants other scores

Hi. I have a set of likert scales where on some items my participants have selected 'PFNTS' my Phd supervisor has said that as long as more than 50% of the data points in a scale are there then I can calculate the mean of those scores and use them to populate those missing data points. If there are more than 50% missing then I will leave them as such and disregard that individual from that particular scale analysis.

I have found loads of syntax for replacing these individual data points with the sample mean but that doesn't work for me theoretically. I have also found syntax describing how to do this to compute the final score on the scale but I need these individual datapoints populated as I am doing a regression on them later. Does any one have syntax for this?