Incomplete questionnaire data - replace non-response with pro-rated score


I want to calculate a total score of a questionnaire with a couple of missing items, but want the missing data points to be included as the mean of the specific case/person's scores on that questionnaire only, rather than the dataset mean for that questionnaire. Is there a way to code this for SPSS syntax?

For example...
SUM.3(Item_1, Item_2, Item_3, Item_4, Item_5) .... IF item missing use MEAN)

For some questionnaires I am using, the specific instructions to replace missing items is to include the mean of 4 specific, related items... is there also a syntax to calculate and include this figure in an equation that calculates the sum of all items?

This is just an example and I have several questionnaires with many more than 5 items that might have only one item missing and without their total there is now a lot of data missing... so if there was an efficient way of doing the above I would be very grateful to find out about it.

Thanks for your help!