Paul,
You can't fix it through the oval inspection cover. The gearbox cover will have to come off. The springs are available, but it may be a case of the spring not engaged with the starter quadrant (the partial sprocket that engages the gearbox main-shaft pinion behind the cover). If the spring is not engaged, it could be a case of the previous owner using the wrong spring, or, less likely, the wrong quadrant.
Richard L.