  Elastic $K^+ n\to K^+ n$ and charge exchange $K^+n\to K^0p$ reactions at low
momenta are investigated using the partial wave analysis. Isospin relation
gives constrains for the partial $s$- and $p$-waves among elastic $K^+p$,
$K^+n$ and charge exchange $K^+n\to K^0p$ amplitudes. Two sets of the phase
shift for these partial waves in the isoscalar channel are obtained from the
fit of experimental data on total and differential cross sections. Polarization
observable leads to a good criterion to decide which set is valid. Differential
cross section data near $P_{\rm Lab}=434$ MeV/$c$ suggests a possibility of a
resonance, e.g., the exotic $\Theta^+(1540)$ baryon around $\sqrt{s}\simeq$
1.54GeV.
