Probably just a standard caravan wheel. 13inch 4 stud is likely to be either 100mm pcd(distance across stud centres) or 5.5inch pcd. Your local caravan shop will stock them or you can usually find them secondhand.
Rim width will probably be 4.5j with around 30mm offset. this should be stamped on rim. It will probably look different from original but if it fits it will do as a spare.
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Hi, if I remember correctly they are the same as a ford 100e wheel visit poppartsplus.com they list 2nd hand rim £15, you will just need to get them to check hole sizes and spaces for you