Go to a title and tag place and get an application, its a few pages. I think you are getting what is referred to as a reconstituted title. You'll need to have pictures from each angle, inspected by a certified trailer guy to have a Gross Weight applied, you'll have to list all the materials and costs in the construction, possibly with receipts for the materials. It is a major pain and can be pretty expensive.
If you have friend in NJ, have him to it. The MVC will issue a HOM serial number and give him a title and registration w/ a lot less hassle and no inspection. Then 'buy' it from him and re-title it in PA.
We did this w/ an abandoned tractor trailer back b/4 the price increases and it was $4.00 for the title and $12.50 for the trailer registration/plate. It is still under $100.00 even w/ the latest increases.
NJ doesn't always title boat trailers; just the boat. Usually the seller gives a bill of sale w/ the boat listing the trailer's info. I had a co-worker who bought a old runabout to fix up and had access to a trailer w/ no title. He was able to get a 'homemade' title w/ out having any hassle.