Given the "chassis" is already existing then it is prior art in terms of your patent. The tricky question is whether only 5/6ths of your concept is patentable, or the whole lot of it.
If the "chassis" is only an extant example of a component required to make your concept functional, then generally the whole concept would be patentable, the relevant claim needs to refer to that part by its function, and note that parts of that type are currently available. The only time it would get difficult is where there are (a) specific features of the "chassis" that are claimed in the original, and (b) these features aren't obvious to "someone skilled in the art" , and (c) you must have these features.
The original patent needs to be referred to in "prior art", and in the description section you can identify the specific part. A sloppy claim for the "chassis" in your patent may make it easier for others to circumvent your patent.
For example you might be using the "chassis" for your prototyping and experimentation because it's convenient, but in mass production you would make your own parts with a similar interface. You wouldn't think to make your own screws and nuts and washers, and ignition coils or pumps or plumbing connectors, at some point you have to draw the line between buying sub-assemblys and making them in-house.
A secondary issue arises as to whether you could actually make and sell your product. The actual spare part may not be available on the market to buy. The original maker might raise the price. The original maker might sue you for "defacing" his product. If you make an identical copy of the part then you will be breaching copyright. If you make something that is functionally similar to the "chassis", but retains say the dimensions of the mounting holes, and otherwise designed by yourself from the ground up and looks vaguely the same from a distance, then copyright is not infringed. The functionality of the "chassis" is irrelevant from a copyright perspective.
You need to draw a line between inspiration and imitation, so you can't patent tractor tubes as pool toys, but inflatable donut shaped pool toys could be patented (assuming no one has been seen in public using tractor tyres in the pool prior), even if they are the same dimensions as a standard tractor tube.