I have a couple of Meoptas. One is an Opemus 6 with a Meochrom Color-3, the head one model newer than yours I think. Which Meochrom version-number do you have on your Op-5?
It looks as though the same mixing box is used for Meograde and Meochrom and Heiland - the difference comes in the filtration rather than the mixing - and there are different designs for 35mm and 6x6 negatives. These boxes are expanded polystyrene with no coating or colour, with a diffuser lens and mounting flange from hard plastic. That material is even used in Leitz Focomat V35s so should be ok from an operational point of view. It may be that someone has previously attempted to modify your enlarger for some reason. The new boxes are 15 Euros, say $20, but you can send a small picture to FotoImpex to confirm the right part. They are pretty helpful.
So far as I ever noticed, both Meochroms I have used (the current one and an older version on an old Magnifax) have 12v halogen bulbs with inbuilt reflectors, running off a transformer unit. Your description of a separate reflector sounds very odd - is this perhaps a head which is much, much older than the enlarger, or even one which someone has hacked to run on mains-voltage in the past?
An item which does need replacing every now and then, and which can cause current and colour-temperature problems (and indeed fire!), is the connector for the halogen-bulb but these are a standard part available from any electricians shop.