We have installed 100's of upgrade kits on that model. It works very well.
It involves the following:
- Insulation panels on left outer side of firebox to limit heat transfer into valve / control area.
- Relocation / protection of plastic components
- Drilling holes in bottom shell to allow air flow to valve cavity.
- New refractories that allow more air flow to valve cavity.
Make sure you follow the instructions 100% to the letter and it will fix it every single time. I have found a few where one step was done wrong or missed and then the MLT melts again. Allow 2-3 hours to install the kit. You need TWO parts, the upgrade kit, and a new refractory panel set. They should give you warranty credit for both of them.
Let me know how it goes, good luck.