Thats odd dirt bike chains are often run even looser because of the long travel suspension. You may be able to run two master links and a one extra link but that will probally be too long. If you have a chain riviting tool you should be able to make it long enough. I would take it back to the shop and make them fix it. I don't know if I would want a shop that let a bike out the door like that working on my bike again though.
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1995 A kit TZ 250
1994 Rothmans NSR 300sp SOLD
1998 RM 250 freestyle/motorcross
1998 Meathanol burning CR 500 AF super retard/motorcross/sand dunes/play bike