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Thank you for input! I will start with the stock servo arm & once it breaks I'll probably design a servo saver out of resin with higher tensile strength. My plate looks different from the one you worked with. I'll see whether I can design around it. The screw studs get in the way of using the tape method, unless I clip them off. I may just redesign the entire upper gear housing plate with a bracket for a modern standard servo unit. Resin printer is down for the moment & I'm out of PETG filament. I'll be putting this on hold until I solve one of those two issues. In the meantime, I've loosened the sprocket pinion gear on the old drive motor by prying with a screwdriver at first, then following up with a needle nose pliers to step it up off the shaft. EDIT: As I've already purchased the Hobywing EZRUN, it appears there is a difference in shaft size (stock=2.3mm, EZRUN=2mm). I'll either source a new brass pinion gear for a 2mm shaft size, or drop a plastic one in until it burns up.

Nikko85 inspired me to take a similar approach to restoring my own Hot Machine: I received mine as a childhood gift & determined about a year ago that I would upgrade it to work with a modern MCU. I'm planning on installing an F405-WSE & using Ardupilot, or similar. Nikko found that a standard metal servo made a great replacement for the stock micromotor which acted as a servo arm. I've received my replacement servo today. The replacement came with a servo saver, but it's going to be too big. I have access to 3D printing hardware & plan to use the old arm with a new plate to adapt it to the replacement servo. The next challenge will be finding a solid way to secure the servo to the top gear housing. Not sure if I'll use tape or create a bracket to secure it.

Thanks so much for reply & suggestions. The F405-WSE would be an attempt at future-proofing it somewhat. I'd have more software control over more servos & lights than an AIO receiver module. It'll be a true project RC car. I plan on adding more to it as time goes on, but I want to start by just getting it running with modern motors & a LiPo (or Li-Ion pack) first. Will post to share results in a new thread once I see working results. I expect it to be some time yet, but I want to at least get it reassembled as its laying in pieces at the moment. Thanks again. Will return with more as time allows.

So glad I found this. I'm in the middle of updating my own RS Hot Machine from my childhood (different version though, but not that much different). Any updates on this? I'm planning to spend my afternoon taking measurements & putting together a shopping list for a servo & motor. My plan is to put an F405-WSE in it as a controller unit.