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Complete rifles without the mum can be aquired for around $100 if you're patient. I bought what turned out to be very nice Type 99 for $90 at the same auction. Has a ground mum.
Ideally, you want to match the parts to the rifle you have. Look for the small arsenal marks on it and try to match replacement parts to them. If you only have $40 in it, you got a lot of wiggle room, if the bore is ok.
Ebay is a source for stocks and parts, there are other sources out there too. The stock is your biggest cost but I see them in the $80 range quite often. Don't buy the first one you come across unless the price is right, others will come along for the price you want.
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When you check the bore you won't see nice square rifling like we're use to seeing in US made arms. The Arisaka rifles used a rounded Metford style of rifling. I know a lot of people thought a Japanese rifle they were looking at was "wore out", because of the rounded rifling.