Yokomo has listed an MS2.1 conversion kit. Here is how to read a conversion listing

A kit is a chassis you build and fit your own electronics to. An RTR (ready-to-run) arrives assembled with electronics, usually with a transmitter. A conversion kit is neither: it is a bag or box of the parts that differ between two versions of the same chassis, sold to owners of the earlier one so they do not pay twice for the parts that did not change.

That means a conversion kit is, by design, incomplete. It assumes you are supplying a car. The useful question is never "is the new version better" but "which of the parts I am about to pay for do I already own, and which parts of my current car does this kit expect to reuse?"

Also worth separating in your head: an essential part is one the converted car cannot run without, and an optional upgrade is one that changes how it behaves. A conversion kit can contain both, and the listing is the only thing that tells you which is which.

1. Confirm the donor car, exactly as the maker writes it. This listing says it is for MS2.0. Version numbers and spacing matter when you are ordering, and a chassis one revision off can use different mounts or different hole spacing. Match the maker's own product name character for character, not the short name people use in chat.
2. Read the parts list against your own car. Three piles: parts included, parts you reuse from the current chassis, parts you have to buy separately. Conversions commonly leave out the third pile — screws, bearings, shocks, a spur, hex adapters — and that is where the budget quietly moves.
3. Work out what happens to your electronics. Motor, ESC, servo, gyro, receiver and LiPo normally carry over, but the mounting does not always. A change in motor position (front, mid, rear, low or high) or a change in battery format between a shorty and a long pack changes where things bolt down and how the car carries its weight. If the listing does not say, ask the shop before you order rather than after.
4. Count the hop-ups you already fitted. Aluminium arms, knuckles and steering parts from the earlier version are the ones most likely to be superseded by a conversion. If a conversion replaces a part you upgraded last season, that is real money you are spending twice.
5. Check whether anything you run in is affected. Some tracks and series set a spec tyre or limit which chassis can enter a class, and those rules are the venue's or the series', not the maker's. If you run a class with rules, read them before you convert.

• Treating a conversion as a complete car. It is not. If you do not already own the donor chassis, the conversion is the wrong purchase.
• Assuming a conversion is always cheaper than a new kit. It usually is, but once you add the loose hardware and the parts you have to replace anyway, the gap narrows. You can only do that arithmetic from an actual parts list.
• Expecting a new version to fix driving problems. Geometry and weight changes move the car's behavior; they do not substitute for steering angle, ride height, shock oil, gyro gain and tire choice being sorted for the surface you drive on.
• Half-converting. Fitting the new parts you like and keeping the old ones you do not want to touch can leave a chassis that matches no published setup sheet, which makes every later problem harder to diagnose.
• Starting the build before the small parts arrive. Order the screws and bearings in the same basket, or a car sits in pieces for a week.

The listing is on Yokomo's own store: マスタースピード MS2.1 コンバージョンキット (MS2.0用) on teamyokomo.com. Yokomo's broader drift range sits on its drift car pages.

The next useful step, if this is a conversion you are considering, is to wait for the full parts list and instruction sheet on the maker's page and read it beside the manual for the car you already own. The two documents together answer the questions the product title cannot.