The engineer can stamp up your CRS using a stamp bearing his Licence Number if the Stamp is listed in the 145 MOE as being acceptable to the CAA.
If he is working for an M3 he/you are okay.
The CAA tend to like the 145 AMO Stamps to have the 145 approval reference on it (e.g. UK.145.00999) and the Approval Ref of the engineer (e.g. FLS905), but it is possible for the CAA to accept a pre-existing stamp/pattern. Many Stamps for M3 organisations had personal Licence Numbers on them (e.g. AMEL 24693) and this may be carried over in a 145 conversion if the CAA accepts it.
Best to ask!