You cannot shorten J’y suis jamais allé, which is already a short version of Je n’y suis jamais allé, to J’ai jamais, Je suis jamais, or J’y suis jamais alone.

If you really want a very short version of it, you might simply say Jamais allé.

If you want to use J’ai jamais, the closest sentence would be J’y ai jamais été, i.e. Je n’y ai jamais été but beware using être for aller, despite having been constantly observed in French, can be controversial (see http://www.academie-francaise.fr/questions-de-langue#34_strong-em-tre-pour-aller-em-strong )