I have the orange version of this. Love this colour too. And you have the three speed instead of two. Nice!
Id check for play in the crankshaft too. Does it move freely? Id try and flush out the engine to see if the missing circlip bits come out, you dont want those moving around in your cases.
If its been sitting for 20 years its probably best to rebuilt it with fresh seals and bearings anyway so you could opt to take it apart (edited)