I cannot say if it any better but I have used the same Hitachi gear reduction mini starter for 18 years without any problems at all. There are knockoffs of this design and of others as well. The quality lies in the electrical field windings and insulation used. The OEM Hitachi uses an epoxy coated armature and sealed ball bearings on each end of the armature and a large bearing on the flywheel drive gear. Tilton uses them for the majority of their starter appplications.
If the design you have is a permanent magnet type (which it may), the intense heat could be affecting the permanent magnents over time. Have you considered wrapping the header tube(s) in the immediate vicinity of the starter to reduce the radiated heat?? Heat will increase the electrical resistance within the starter which may be part of your problem too.
