As Brian Drummond and JRE have already explained, your calculations seem to be wrong. Maybe you should read some more about battery circuits and how to calculate voltage capacity?
Further, you can edit your question, and update it, rather than asking it again.
I apologise for using some 'shopping' information in my answer, but it seems to be a direct and simple way to deal with part of the underlying motivation for the question. By using ball-park costs, I think this answer can be more helpful.
The motor requires 48V, 1000W, and hence about 21A.
Assuming a LiPo voltage of 3.7V is okay, though it can be 4.2V when fully charged, so be aware of this.
It needs about 48V, and so
48v/3.7V = 12.97, i.e. 13 cells, or 13S
48V/4.2V = 11.43, i.e. 11 cells, or 11S
So 12S is near. It is divisible in several ways, and so it is a reasonable voltage and cell count basis.
Be aware, that would be a peak of 12 x 4.2V = 50.4V, so check your bike's capability.
12V SLA batteries can exceed 12V when fully charged, so it may be okay, but check the spec of the motor-control on your bike. A quick check with a multi-meter to measure the existing voltage across the fully charged SLAs would be a sensible precaution too.
If there is any concern, use no more than 11S, though that is less convenient to build from multiple batteries.
A LiPo able to supply approximately 12Ah, is likely to be able to discharge at significantly more than 21A, but also bear this discharge current in mind when selecting the batteries.
There are lots of web sites selling LiPo batteries.
As an example I looked at HobbyKing which has a battery selector.
I haven't found a 12S with anything close to 12Ah. YMMV on other purchasing sites.
So I tried 6S (22.2V) with a capacity over 6Ah. This would need two in series to reach, approximately, 48V. That battery selector found about 8 candidate batteries, over 10Ah, with a high enough discharge rate. For example, I found a 6S 12Ah LiPo for about £90 inc VAT. Again, YMMV.
So, I think it is straightforward for you to find candidate batteries.
However, it is very important that LiPo batteries are never over discharged.
You will need to check that your bike can protect the batteries. If it comes with SLA, then it might not have that capability. Then you will need to build or purchase a LiPo battery-discharge protection system, capable of handling 12S voltage and, at least, 1kW.
You will also need a good battery charger able to recharge 6S 12Ah LiPo batteries. The batteries are likely capable of being charged at over 1kW. I couldn't see such a beast at HobbyKing. The best I saw was 300W, and that was about £100.
Summary: I agree with Brian Drummond. Batteries with comparable capacity to the existing SLA's, plus discharge protection, plus battery charger, is likely to be in the £300+ area. Looking at the cost of batteries alone ignores significant costs, and hence is not realistic.