It all depends on the understanding of modeling that the giver has , and the expressed appreciation of the gift , or lack there of that gets expressed as " oh wow , but the paint , and body style is wrong " . If left as is , and run as is , the giver sees no sadness , and feels you are happy as a clam . The giver then is clam happy as well .
However if you expressed with glee that you will have sooooo much fun doing some prototypical modifications to a model you were already planning to get some day , then get down and boogie with a paint brush .
You need to read the givers personality , and choose your words carefully . In the long run try to swing future gifts into cash offerings which then can be pooled , and you can get the stuff you really want .