The battery holder of the Metz 45 uses two different contacts from the holder for the rechargeables.
There is no replacement holder for rechargeables. There is of course the complete replacement rechargeable battery pack. It is NiCad. These packs do not have eternal life. There comes a moment you want to replace it. It costs about euro 75 and is still Nicad not NiMh's. Now an option would be to replace the NiCads for NiMh cells. But you would still be left with the totally inadequate charger.
A better option: use the holder for normal batteries, not rechargeables, but load it with NiMh's. You can now use these in a good charger like the Ansmann Powerline 4.
Iit comes to mind to use the other contact. It is not much faster. I did expect it to be faster, but this is hardly the case. But I reckon the flash gun will function better with this modififation.
The pictures tell the story:
I cut a small strip of metal. Folded it in the shape you see in the picture. It now connects the old contact from underneath, directly to the unused contact hole. The regular contact must be isolated very well with some tape.

I do not know if Metz will approve ;-). If you convert your holder in this fashion, you obviously do so on your own risk. In general flashguns are no toys: the currents from the flash condensator can be large enough to kill you.

links:

Metz Blitztips - Everything about using a portable flash unit. It has a complete course in german and english. Nice examples. Good explanations. No direct link anymore: they keep shifting their site around. Recommended
Elinchrom Flash Tips - Joe Craig. Flash in the studio. Joe is no Jost Marchesi ;-) Att: IE only.
High-Speed Flash Photography For Amateur Photographers - Loren M. Winters. Here is the trick with the tape recorder to fire your flash.
High Speed Photography - Andrew Davidhazy. classic
Electronic Flash Information - Toomas Tamm. Very extensive, rather theoretical flash and flashgun - FAQ. He also maintains a good Canon instruction.
Don's Lighting Info Center - Don Klipstein. Lots of information related to many forms of lighting.
Basic and advanced theory, and other wisdom including troubleshooting and repair hints. Info on incandescent, halogen, fluorescent, compact-fluorescent, and also HID lamps such as mercury, metal halide, high pressure sodium, and short arc lamps; neon, low-pressure-sodium, oddballs such as green neon lamps; various UV lamps and carbon arcs. Build or hack your strobes. Lots of circuitry. On the same site is:
Sam's Strobe FAQ - Samuel M. Goldwasser. Notes on the Troubleshooting and Repair of Electronic Flash Units and Strobe Lights.