Can an EASA Part-145 / Part-M Subpart F approved maintenance organisation fabricate their own parts?

Yes, but it is limited.

The approved data necessary to fabricate the part are those approved either by the Agency, the TC holder, Part-21 design organisation approval holder, or STC holder.

EASA Part-M.A.603 (Subpart F) (c) and EASA Part-145.A.40 (b) III indicates that an approved maintenance organisation may fabricate, in conformity with maintenance data, a restricted range of parts for the use in the course of undergoing work within its own facilities, as identified in the maintenance organisation manual.

These own fabricated parts may not sold and can only used for the purpose of the maintenance organisation.

Example of the restricted range of parts are:

(a) fabrication of bushes, sleeves and shims,
(b) fabrication of secondary structural elements and skin panels,
(c) fabrication of control cables,
(d) fabrication of flexible and rigid pipes,
(e) fabrication of electrical cable looms and assemblies,
(f) formed or machined sheet metal panels for repairs.