Some models are, in my experience, not something I want to work on because they are costly for the customer and frustrating for me. Usually at least one of the following applies:
I don't want to deal with the above issues because doing so takes extra time. That time makes the job cost more to the customer, which is frustrating to the customer. In addition, I end up under-charging for the time involved because the bill was so high, which is frustrating to me. That extra time also impacts my already too long backlog because jobs take longer than expected.
I realize that there will be exceptions to the following list. If you have a model that is listed below and think your model may be an exception or you are not sure whether your model belongs in the below list please contact me so we can discuss the model and your job.
While there may be exceptions, diecast steam models do not have sprung/drop out driver wheelsets, making them more difficult and costly to work on.
These models have value to Japanese collectors, so unless you are really really attached to the model, you may be able to get a good price for it.
On customer request I have attemped to modify existing drives. Modifying existing drives can become a rat's nest very quickly, as fitting the new part or subassembly into the system that the manufacturer developed requires careful planning, measuring, and fitting, all of which increase cost. Those attempts turned into drive replacements.
If you want a drive system modified, we need to carefully evaluate the cost-effectiveness of the change(s) you want. In any event, we should talk about what you want done. I need to see the drive system that you want modified before I will take on the job. If the modification involves modifying die cast parts such as gear boxes, I strongly recommend that you consider replacing the drive system instead.
If you have a drive system repair chances are it will be low cost and done with a quick turnaround.
Some scratch built models (other than by me) were built without any provision for a drive system. Powering such a model can be either simple or difficult. Please contact me to discuss.
Here is an example. The scratch model was complete. However the driver bearing slots weren't square (needed for the drivers and rods to work together) nor were the side rod crank pin holes correctly located. Lots of shimming of the driver bearings and re-bushing the side rods was required. All on top of the custom drive bill. Needless to say the customer was shocked at the bill.
Some budget priced models are built inexpensively and can be expensive to work on. For example, on one model the manufacturer did not tap the holes that were drilled for screws - the screws, which were not self-tapping screws, were used as self-tapping screws in order to avoid the cost of tapping the holes. Well, that gets a screw in a hole. Now, remove that screw if you can, and if you were able to remove it without damaging/destroying it, now attempt to run it back into the hole. So when I run into this, I have to take the extra time of going over the entire model tapping holes as needed. Sure, I could skip that but that flies in the face of how I view my product quality.
Sorry for the negativity, but I have learned from bitter experience that fitting custom drives into some locos, and some drive system modifications can have a very high cost.