I have an old Scott Capa bike. My gears do not engage properly, sometimes when cycling the pedals just spin and gears do not "rachet" when pedalling backwards. I'm told that I need to replace the freewheel/cassette. Firstly is this likely to be correct? Then if so the current part is a Shimano CS-HG30-81 but these do not seem common - coming up as vintage - is it possible to replace with a different modern part and if so what is a good cheap alternative i can get from Amazon or similar. Thanks in advance for the advice for a complete bike novice.