SD (dont@reply.com) gurgled happily, sounding much like they were saying

MA NG and MA NA are the chassis number series identifiers - you really want
to quote the engine series identifier from the engine plate - it'll be
M25/6xx

However, I'd think that all the (petrol non-turbo) 2.5ie exhaust valves are
the same, possibly even the same as the 2400. Are they not available from
dealers any more?

Try ringing round some of the dealers in your area that have been in
business for years - I got a complete set of valves, rockers and cams for an
Ami Super that way, and the dealers were so glad to get rid of them that I
got 'em all for big discounts!