FSE 450 rebuild, DR-Z parts?

[V1T0]

Hi! I'm rebuilding my 450 engine. I have to change these parts:

OIL PUMP BODY
OIL PUMP ROTOR
RETRIEVABLE PUMP RING
RETURN OIL ROTOR
OIL PUMP COVER
CENTRAL CRANKCASE GASKET
CLUTCH COVER GASKET
DISCS COVER GASKET
IGNITION COVER GASKET
CAM SHAFT DRIVE CHAIN
CYLINDER GASKET
CYLINDER HEAD GASKET

I also need bearings, piston, cylinder regeneration, cam regeneration, clutch discs and tappets.

And I have a few questions.

Where should I order parts? My mechanic told me that he can buy parts for me and it will be 1500 euro which is lots of money.

I heared that cilinder head is exactly the same as Suzuki DR-Z 400, is it true? OEM tappets would be 200 euro, tappets from DR-Z are much cheaper.

And the bearings. Can I use others than OEM? Like SKF, KOYO or something? What's the difference? I need 6306C3 and 6303C3.

Thanks.