personaly, for the standard builds, (romy, etc) I like the receivers form www.globaltrades.com
YOu can get them parkerized for only $65 + ffl fee
The suomi has a nice drum. There are some semiauto parts, receivers, etc for sale too... bonus...
I have seen an 80%, and oyu can use AK or AR trigger gorups, for a legal semi auto.
the drums are very very nice. REad up on them, best in WW2 I do beleive.
EDIT, suddenly, a thought comes to my brain. The 74 bolt head is the correct size.. one could use it by drilling a hole in the original bolt, and droping the bolt head in there. Then use a AK TG. Just a thought. And you will need to weld a block in there, so a standard open bolt can't be used. AKA, a regular full auto bolt can't fit. and send plans to the alphabet boys before oyu build, to make sure they like it. end of disclaimer.