many ways to build a dredger. Depends on how much material you want to move.

First you need to figure out if you need an auger. Most likely yes. An auger will be something to churn the mud up. After the mud is churned up, then you would need to pump the muddy water away. You need a pump for that.

Building a dredger is a serious task. You need to figure out floatation, propulsion, pump rate for water flow, and auger size.

With the price of metal, I would hate to see what one built with scrap would cost.

Look around for dredgers. There are pictures that will lead you in the correct direction.

What kind of dredger are you looking for? what kind of material? what kind of volume do you want to move?

Yeah more specs would be needed but dredges are fairly easy to fab if you know the measures you want to deal with.  Also motor placements and balances are critical.   Been dredging for 18 years and I'm only 25.  have built 5 of my own dredges.  I'll be glad to help,  hence my alias  4theluvofgold