[HORN_N_CA]

Hey folks, newbie here and wanted to ask a few questions, see what kind of info I can get from ya'll.
My Dad recently gave me his Marlin Model 99 M1 that he bought new back in '67 or '68. It's in pretty pristine condition, he always kept his guns in great working condition. The only thing is it hasn't been fired in probably almost 40 years and only been taken out of it's carry bag maybe a dozen times just to look at it over those years.
I would love to clean it and fire it, but a guy at a gun shop I went to, to buy a cleaning kit said I should have a professional look at it, as a firing spring? may be worn out or bad and could mess it up if fired. I'm wondering what ya'll out there think. Having not been fired or abused, etc, how things like that would just go bad, but who knows.
Also I discovered the Numrich website for parts, but alas the part I do need, the "Inner Magazine Tube Assembly" is missing after a Dad's friend screwed up and had it fall out, years ago,which was probably the last time it was fired. Obviously it can be fired a single round at a time, but I'd like to get that replaced. Is there any other places ya'll suggest to get parts for these?
And lastly, it's old enough it doesn't have a serial number, but there is an "LK" on the barrel below and to the right of the imprint of the name and model on the barrel. Anyone know what this means?

I can upload pictures later, it's a great looking rifle I think.

Thanks!

Horn