The stock return spring is part of the brake linkage set, part# 86356. Plenty in stock on ebay and amazon right now.
https://www.amazon.com/HPI-Racing-86356-Linkage-Savage/dp/B000E46F5G
The engine is not stock, looks like a hot bodies engine. Neither is the filter. Assuming that other components are stock though (like the exhaust, shocks, TVPs and radio box), that is a Savage 21 RTR.
The Savage XL has a more reliable reputation. Do some looking around, including multiple threads on this site, the Octane has some serious short-comings. Not to say if you get an Octane that it will definitely have issues, but it seems more likely.
Welcome @Savagebeast93!
To be honest, even if the tune isn't necessarily your issue (and it very well could be), I would still re-tune it from factory settings. Probably replace the glow plug and fuel lines as well because your don't know how new they are.
Not sure how much that roll...
Been running Robinson racing spur and clutch bells for over a decade without issue. Perhaps this is a recent quality issue? I don't think I've read of other complaints.
Can you elaborate on what else your conversion consists of? Are you using all stock Flux XL parts? Or some aftermarket?
From what I can see, the stock Flux XL uses a 16T pinion and a 43T spur. I'm guessing your gears are simply too large for the application, stock.
If you have money to burn, Tekno still makes Nitro kits. Great kits, but they cost alot and you'll need to supply your own electronics and engine.
https://www.teknorc.com/shop/tkr5406-nt48-3-18th-competition-nitro-truggy-kit/#tab-detailed-description
Actually, I take that back. I haven't looked through that in quite some time. The sad part is that I think the only RC on that preview that hasn't been released (it took them quite a while to get most of those out) is the one you want, the new Savage X SS.
I think what you were looking for was this:
https://www.hpiracing.com/en/article/view/2017010401
Sadly that, along with most of what was previewed at the Nurnberg toy fair that year was never released. HPI went through bankruptcy and then was bought.
To my knowledge, there hasn't been a Savage kit (aside from the Savage XS SS kit, but that's the mini savage) since the Savage X SS. It has been discontinued for quite some time. Kits are becoming rarer and rarer and almost nothing is available in kit form.
Have you made sure the adjustment nut on the steering bell crank is tightened? Stock adjustment is shown lower right of the picture below. I personally would go tighter.
https://www.hpiracing.com/en/part/87039
Go through the manual for the reverse module. It gives all the original part numbers for the components that were removed in order to fit the reverse module.