Can't help getting crucified, I know...

I got this frame, a Mongoose DX4.0, that I was thinking of buidling up; wanted to check some prices on ebay for the bootom bracket, crank and fork.

My question is how cranks, for example are identified: the frame is threaded, 38mm ID x 68mm width. I see cranks on ebay that are 68x108mm - is this diameter x width?
Are the cranks identified as 1-3/8"?

The fork stay (not sure of the terminology here) is about 34mm ID and 100mm tall; not threaded. Is this a 1-1/8" fork?

The rear stay is 135mm wide; don't even know where to go with this one.

back to school time?

For bottom brackets 68 x 108mm translates to 68 bb shell width (standard)
and a 108mm spindle. The spindle determines chainline.

And with the rear triangle spacing, you'd need a 135mm hub, or whatever with spacers to fit.

Also chainline depends on your rear hub and bottom bracket spindle length.
So find a hub or wheelset before choosing cranks and bb.

You're going to be looking for plain old JIS square taper cranks. Using mountain cranks I'm assuming.
Sounds like a plain old 1 1/8" threadless fork to me. Standard parts.

The mountain fixed/SS rear hub will be a moderate challenge. This isn't really a mountain bike forum but there are some fixed/SS mountain riders here, somewhere.

So, I should be able to use some old mountain bike wheels and spacers to do a simple single speed conversion, correct?

If I do this, how do I determine the crank required?