I haven't found a good alternative to mentaiko for this. Ikura is too big, and roe like tobiko (flying fish roe), masago(capelin roe), and kazunoko (herring roe) have a poppy texture that doesn't work great for this. Bottarga made from mullet roe will work if it has not been dried, but you will need to change the ratio of ingredients if you use dried and grated bottarga.