Assuming....
db.define_table('mytable',...)
and the id of the source record is source_id, you can do:
# get the record
row=db(db.mytable.id==source_id).select()[0]
# set the default for all fields by ID
for fieldname in db.mytable.fields:
if fieldname!='id': db.mytable[fieldname].default=row[fieldname]
form=SQLFORM(db.mytable)