Yes, it is OK to lay an SLA battery on its side but you need to be aware that it is denser at the bottom than at the top so the weight is not evenly distributed and you will either have to mount it off centre or provide compensating ballast at the terminal end.
If you have not already bought the battery then a NiMH pack is a better option as you will get twice the usable capacity and running time for approximately the same weight.
Colin