Hi and welcome to mightyram. The old mechanical lifter head has different specs to the hydraulic head - the components aren't cross compatible. The Astron I engine you have didn't have a removable side cover. The updated engine is referred to as an Astron II and also uses a different intake stud pattern that is compatible with 3 intake set ups - carb'd, a single injector ECI, and MPI in countries outside of the US.