SLMG is a national rating, TMG is a JAR-FCL rating.
As you have never flown a motor glider, your SLMG rating will be well out of date. So, either:
1. Do the differences training on whatever the aircraft is and fly it on your SEP Class Rating (althought the hours won't count towards SEP revalidation requirements.
2. Take a TMG Class Rating LST, add a TMG Class Rating to your licence and then keep [b]both[/i] SEP and TMG ratings valid by flying hours on either to count for both. See LASORS:
[i]If the licence contains both SEP (Land) and TMG ratings, revalidation requirements of both classes may be satisfied by completing the requirements in EITHER a TMG or a SEP (Land) aeroplane (or a combination of both).