You can replace the solenoid on it's own but if the starter is playing up it will almost certainly all be nearing it's lifespan limit.
I've fitted a 'WOSP' hi-torque geared starter to my car. 'Powerlite' also produce a similar one. These are geared so they turn your engine quite quickly which helps a lot on a slant 4. My WOSP failed almost 2 years after purchase due to a bearing fault and was replaced FOC without any hassle at all. WOSP starter for a Vauxhall slant 4 is LMS042 1.4kw.
If buying a standard one from sources on eBay etc be sure that it mounts at the correct angle. The solenoid must be vertical, starters fitted to CF vans and Victors can have the solenoid at about 45 deg to the vertical. This may foul on the steering column on a Viva. Also uprated 'standard' Lucas type ones are 1.8 Kw but still turn over quite slowly and will drain a battery very quickly.
Incorrect - may foul on steering column
Correct