Ah I was wrong, the Rekord 120 has a monolux unit rather than a separate light bar.
The best thing I can think to do to really boost your lighting would be to rip the whole old light unit out the hood, and put in a compact T5 stater unit and bulb - or better yet a twin unit.
If you got the interpet twin 55W unit and tubes would cost about £70-80 to do and would give you over 3.6 WPG (since they are T5's) - so you would most likely need to inject CO2.
If you got the interpet twin 36W unit and tubes would cost about £60-70 to do and would give you over 2.4 WPG.
If they get hot, it would probably also be a good idea to buy a T8 reflector and mod it to fir the T5 tubes, or install cheap PC fan in the hood (would cost like £5), but you may be fine.
You could also do the same with regular T5 starter units for less WPG, or use single compact T5 starters, but the single compact ones wont give you the most even distribution of light.
The T6 tube could fit straight into your tank, but you would probably need the brushing set (also on aquaeesentials since it's a juwel tank) to keep the seal water tight. Though realistically the T6 tube wont make too much difference.