I would look for a reputable
axle shop or a place that makes and sells utility trailers (the kind landscapers use). Let them do the measuring and ordering. Rubber torsion axles are custom made to spec, so if you measure it yourself and make an error, you are out the cost of the
axle.
The two main suppliers I am familiar with are Dexter and Lippert. On the 16' models,
Scamp uses a Dexter #10 3500# axle with a 22.5 degree down angle and 10"
brakes or the equivalent Lippert.
This would be a good time to inspect the frame very carefully. Older
Scamp 16's are prone to frame weakness and failure under the front street side corner where the A-frame bend starts.