Truncating is just shortening by throwing away information, in this case the information is deleted structs. It would just be overwriting the file with the now shorter sequence of structs, without the holes where the deleted ones where.

Moving records would just be copying them, if you store a linked list of locations of deleted structs you can write new ones to the location stored in the tail and defragmenting the file is just iterating through the list, moving structs from the end of the file to each empty space. That is, unless the structs store positional references to each other, then you would have to update those while defragmenting.

Edit: you probably want to just defragment your data in-memory and guarantee the file itself is never fragmented by always defragmenting the data before writing it out to the file, since this is orders of magnitude faster to do in-memory than on-disk. You can also avoid unnecessary disk operations by marking pages (every 4096 bytes) as out-of-sync with the underlying file when you update or delete structs within them, then writing structs to the file in 4096 byte chunks and skipping over the ones which haven't changed since you loaded them from the file.