Hello Jepoy,
I can only say that I was able to build an ISIS adjacency over an L2TPv3 pseudowire without any need to adjust clns mtu as I had expected
ISIS uses hellos = supposed MTU so it is similar to using IP packets of size 1500 byte.
Clearly the device needs to fragment the L2TPv3 PDU in order to cross links in the middle towards remote PE node.
It is wise in this cases like when using PPPoE or forms of IPSec and/or GRE to reduce MTU to avoid fragmentation otherwise buffers are stressed to handle the fragments at the other L2TPv3 endpoint.
Hope to help
Giuseppe